I have no idea what’s going on, but either Translate’s source code is quite broken, or the database has been corrupted.
When I try to mark m:Template:Navigation header/COVID-19 for translation, I have the option to turn on translation-aware transclusion (it’s off) and to upgrade to the latest syntax version. However, the syntax version has already been upgraded in October (example edit by FuzzyBot adding the new markup), and the main COVID-19 page’s May 8, 08:35 version (the one before @1234qwer1234qwer4’s today revert) wasn’t full of raw <translate> tags when archive.org saved it a few hours after that May 8 edit, meaning the translation-aware transclusion was turned on at that time.
The translation-aware transclusion could in theory been turned off since archive.org saved the page, but the syntax version update cannot be undone on-wiki, only directly in the DB, so something’s broken for sure.
Translatable page settings are no longer disappearing unexpectedly.
There was a bug, which got more severe recently, which caused a silent and permanent deletion of settings of translatable pages when associated translation page was deleted (see https://www.mediawiki.org/wiki/Help:Extension:Translate/Glossary). Such settings include priority languages, syntax version and transclusion support and other metadata.