I think the issue is parseVersionScriptContent is only passed code lines, so the if the line containing the ELFVERSION marker contains no code, the current line is the next one after that, which means the next line is two code lines further.

Debugging from syncqt on parsing qthread_p.h, with the ELFVERSION marker from https://codereview.qt-project.org/c/qt/qtbase/+/451705 :

qthread_p.h: class QPostEventList : public QList<QPostEvent>
qthread_p.h: ;
line ignored: ;

This is also the reason why QTextCodec is marked as a private symbol:

#else // without textcodec:

// ELFVERSION:ignore-next
class QTextCodec