UTF-8 Encoding errors with Security patch in 2.9.4+dfsg1-6.1ubuntu1.3

I'm using libxml2 via perl-bindings in XML::LibXML.

After upgrading to 2.9.4+dfsg1-6.1ubuntu1.3 my perl-scripts threw many UTF-8 encoding errors, e.g.
"Malformed UTF-8 character: \xb0 (unexpected continuation byte 0xb0, with no preceding start byte."

Re-installing libxml2.so.2.9.4 from 2.9.4+dfsg1-6.1ubuntu1.2 and linking libxml2.so.2 to this file without changing anything else lead to error-free execution of the perl-scripts: All Unicode characters are ok.

At present, I'm not able to reduce the error situation in such a way that a simple perl script can be given which reproduces this error - I'm working on this. I'm also not able to provide an error description using libxml2 directly, without perl. So, please be patient with me and this stumbling bug-report.

My system is:
Description: Ubuntu 18.04.4 LTS
Release: 18.04