I've recently seen this issue on other wikis using the Timeless skin, such as deepsky and the tuscriaturas family of wikis - viewed logged out they use Timeless, but viewed logged in they use Vector 2022. To see Timeless when logged in, I have to change my preferences on each wiki (resetting to default preferences works), even though I didn't previously change any preferences on those wikis and I ensured I don't have any global preferences set.

The upgrade to MediaWiki 1.38 seems to have fixed this issue. I've now removed the wordmark image on the wiki in question.