Weblate has FAQ on the subject:
Why does Weblate still show old translation strings when I’ve updated the template?

Weblate does not try to manipulate the translation files in any way other than allowing translators to translate. So it also does not update the translatable files when the template or source code have been changed. You simply have to do this manually and push changes to the repository, Weblate will then pick up the changes automatically.

Note

It is usually a good idea to merge changes done in Weblate before updating translation files, as otherwise you will usually end up with some conflicts to merge.

For example with gettext PO files, you can update the translation files using the msgmerge tool:

msgmerge -U locale/cs/LC_MESSAGES/django.mo locale/django.pot

In case you want to do the update automatically, you can install addon Update PO files to match POT (msgmerge).
https://docs.weblate.org/en/latest/admin/addons.html#addon-weblate-gettext-msgmerge

we are fixing this issue with help of @misc
it is an automation missing (yes, we should have added it long ago)

And finally, it is now fixed, for real this time! 🎉