A sample .ts file is uploaded. Loading it into Linguist from Qt5/6 yields different results:
Qt5 Linguist: all good
Qt6 Linguist: broken plural rules (see screenshot)

The issue is language tag:

<TS version="2.1" language="en-US">

"en-US" is separated by dash. Replacing dash with underscore solves the problem.

Nevertheless, Linguist should be able to handle both dash and undrescore. Qt6 Linguist is more or less broken. Possible related commit:
https://codereview.qt-project.org/c/qt/qttools/+/343860