XSLT extension elements: self_node evaluated only sometimes

Hi, I am following this example: https://lxml.de/extensions.html#xslt-extension-elements . All good, and it even works if I substitute this line:

<foo><my:ext><child>xyz</child></my:ext></foo>

with this line:

<foo><my:ext><child>xyz<xsl:text>123</xsl:text></child></my:ext></foo>

that is, I get in the output:

<foo>I did it!<child>xyz123</child></foo>

which is great.

However, if instead I do:

<foo><my:ext><child>xyz<xsl:value-of select="text()"/></child></my:ext></foo>

then the output becomes:

<foo>I did it!<child xmlns:xsl="http://www.w3.org/1999/XSL/Transform">xyz<xsl:value-of select="text()"/></child></foo>

which is not what I want because I want this to be evaluated.

This means the XSLT inside the extension element gets evaluated _sometimes_. xsl:text gets evaluated but xsl:value-of does not.

Here your original code from the documentation, in one piece:

import lxml.etree as etree

class MyExtElement(etree.XSLTExtension):
    def execute(self, context, self_node, input_node, output_parent):
        print("Hello from XSLT!")
        output_parent.text = "I did it!"
        # just copy own content input to output
        output_parent.extend( list(self_node) )
        # output_parent.append(self_node)

xslt_ext_tree = etree.XML('''
 <xsl:stylesheet version="1.0"
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
     xmlns:my="testns"
     extension-element-prefixes="my">
     <xsl:template match="/">
         <foo><my:ext><child>XYZ</child></my:ext></foo>
     </xsl:template>
     <xsl:template match="child">
         <CHILD>--xyz--</CHILD>
     </xsl:template>
 </xsl:stylesheet>''')

my_extension = MyExtElement()
extensions = { ('testns', 'ext') : my_extension }
transform = etree.XSLT(xslt_ext_tree, extensions = extensions)

root = etree.XML('<dummy/>')
result = transform(root)
print(str(result))

Here my config:

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)

Thanks!