This is a side-effect of T98014. PHP does language fallback in the header section, but JS does not. So, if you edit the terms of an entity which doesn't have terms in the UI language, after leaving edit mode the header section might look different than it does after a reload.
|Open||None||T124286 [Epic] Wikidata language support|
|Open||None||T76216 [Epic] Language fallback|
|Open||None||T135871 [Bug] JS does no fallback for terms in header section|