Making a null edit to the translation unit page (https://meta.wikimedia.org/w/index.php?title=Translations:Template:Engine/translatable/1/en) fixed the problem.

Thanks, I didn’t know we could edit translation units in source language.

So, I have tried to reproduce this bug, but I haven’t achieved: after all, I don’t think this is a regression.

EDIT: Oh, yes, in fact I did. To reproduce this bug, I have added a TVAR inside a page who was already marked for translation, and I have checked “Do not invalidate translations” when marking the page for translation again.

I cannot reproduce this. Leaving open to test after T254484: Tag untranslated translations units with lang and dir attributes.

Could possibly be also related to T255334: Translation page does not contain the latest translations/last translation.

I just reproduced this issue with these steps:

1. Find a page which has already been marked for translation.
2. Edit it to add a new tvar (e.g. <tvar|foo>test</>).
3. Open Special:PageTranslation with do=mark GET param.
4. Check “Do not invalidate translations” for concerned translation unit.
5. Submit the action clicking “Mark this version for translation”
6. Visit [[https://meta.wikimedia.org/wiki/User:Pols12/Sandbox/en | the /en subpage]]: the tvar name is displayed instead of tvar content.

I have not achieved to reproduce this bug anymore, thank you! 🙂