I have InDesign 2021 and Big Sur. But I don't encounter it because I always have the Application Frame turned on. Your case is with Application Frame is turned off (when I tried to re-create it). Here's what it looks like with an Application Frame (no problem!):
Thanks Steve, that's good info (even though I don't use app frame). I myself mostly don't encounter this issue either as there are usually a few tabs open. My main grievance is with Apple making those title bars white at all!