A family of Microsoft word processing software products for creating web, email, and print documents.
In some previous versions of Word for Mac & for Windows there was a Preference to Automatically Show/Hide the ribbon, but that was eliminated.
Since that isn't what you're referring to, however, that really isn't relevant. What you're referring to is the Title Bar of the document window. The fact is that there is not nor has there ever.been an option in Mac Word to not have it displayed... It's an integral component of the document window & should be displayed in any open document window.
If you're experiencing an intermittent disappearing act there must be caused by something of a systemic nature which may be related to a variety of display issues in Big Sur. I'd suggest using Help> Feedback in Word while you have a problem window displayed & include a screen shot by way of the check box in the I don't like something dialog. That goes directly to the development team.