tostring truncates output when encoding set to "utf8"

I ran into a bug that causes lxml to truncate the output when using "tostring" with encoding set to "utf8", while it works correctly when encoding is set to "utf-8". Running the attached example file produces the following output for me:

Bad:
b'<record><datafield tag="520" ind1=" " ind2=" "><subfield code="9">APS</subfield><subfield code="a">The first measurement of the dependence of &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xb3&lt;/mi&gt;&lt;mi&gt;\xce\xb3&lt;/mi&gt;&lt;mo stretchy="false"&gt;\xe2\x86\x92&lt;/mo&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;+&lt;/mo&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;\xe2\x88\x92&lt;/mo&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt; production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at &lt;math display="inline"&gt;&lt;mrow&gt;&lt;msqrt&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mi&gt;s&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;N&lt;/mi&gt;&lt;mi&gt;N&lt;/mi&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/msqrt&gt;&lt;mo&gt;=&lt;/mo&gt;&lt;mn&gt;5.02&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mi&gt;TeV&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;, with an integrated luminosity of approximately &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mn&gt;1.5&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;nb&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;mn&gt;1&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt;, are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mn&gt;8&lt;/mn&gt;&lt;mo&gt;&amp;lt;&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mi&gt;m&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;mo&gt;&amp;lt;&lt;/mo&gt;&lt;mn&gt;60&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mi&gt;GeV&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt; are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mrow&gt;&lt;mo stretchy="false"&gt;|&lt;/mo&gt;&lt;mi&gt;\xce\xb7&lt;/mi&gt;&lt;mo stretchy="false"&gt;|&lt;/mo&gt;&lt;/mrow&gt;&lt;mo&gt;&amp;gt;&lt;/mo&gt;&lt;mn&gt;8.3&lt;/mn&gt;&lt;/mrow&gt;&lt;/math&gt;. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.</subfield></datafield><datafield tag="520" ind1=" " ind2=" "><subfield code="9">arXiv</subfield><subfield code="a">The first measurement of the dependence of $\\gamma\\gamma$$\\to$$\\mu^{+}\\mu^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\\sqrt{s_\\mathrm{NN}} =$ 5.02 TeV, with an integrated luminosity of approximately 1.5 nb$^{-1}$, were collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region 8 $\\lt$$m_{\\mu\\mu}$$\\lt$ 60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\\eta|$$\\gt$ 8.3. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstrat</subfield></datafield></record>'
Good:
b'<record><datafield tag="520" ind1=" " ind2=" "><subfield code="9">APS</subfield><subfield code="a">The first measurement of the dependence of &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xb3&lt;/mi&gt;&lt;mi&gt;\xce\xb3&lt;/mi&gt;&lt;mo stretchy="false"&gt;\xe2\x86\x92&lt;/mo&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;+&lt;/mo&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;\xe2\x88\x92&lt;/mo&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt; production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at &lt;math display="inline"&gt;&lt;mrow&gt;&lt;msqrt&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mi&gt;s&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;N&lt;/mi&gt;&lt;mi&gt;N&lt;/mi&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/msqrt&gt;&lt;mo&gt;=&lt;/mo&gt;&lt;mn&gt;5.02&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mi&gt;TeV&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;, with an integrated luminosity of approximately &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mn&gt;1.5&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;nb&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;mn&gt;1&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt;, are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mn&gt;8&lt;/mn&gt;&lt;mo&gt;&amp;lt;&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mi&gt;m&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;mi&gt;\xce\xbc&lt;/mi&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;mo&gt;&amp;lt;&lt;/mo&gt;&lt;mn&gt;60&lt;/mn&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mtext&gt;\xe2\x80\x89&lt;/mtext&gt;&lt;mi&gt;GeV&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt; are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range &lt;math display="inline"&gt;&lt;mrow&gt;&lt;mrow&gt;&lt;mo stretchy="false"&gt;|&lt;/mo&gt;&lt;mi&gt;\xce\xb7&lt;/mi&gt;&lt;mo stretchy="false"&gt;|&lt;/mo&gt;&lt;/mrow&gt;&lt;mo&gt;&amp;gt;&lt;/mo&gt;&lt;mn&gt;8.3&lt;/mn&gt;&lt;/mrow&gt;&lt;/math&gt;. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.</subfield></datafield><datafield tag="520" ind1=" " ind2=" "><subfield code="9">arXiv</subfield><subfield code="a">The first measurement of the dependence of $\\gamma\\gamma$$\\to$$\\mu^{+}\\mu^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\\sqrt{s_\\mathrm{NN}} =$ 5.02 TeV, with an integrated luminosity of approximately 1.5 nb$^{-1}$, were collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region 8 $\\lt$$m_{\\mu\\mu}$$\\lt$ 60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\\eta|$$\\gt$ 8.3. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.</subfield></datafield></record>'

As you can see, the ouput of the last subfield is truncated in the first case.

Required information:

Python : sys.version_info(major=3, minor=9, micro=2, releaselevel='final', serial=0)
lxml.etree : (4, 6, 3, 0)
libxml used : (2, 9, 10)
libxml compiled : (2, 9, 10)
libxslt used : (1, 1, 34)
libxslt compiled : (1, 1, 34)