Preview disappears in presentation mode

Hello all. I've seen similar problems posted, but none precisely the same as mine. 

Every time I use Presentation mode, with several different documents, the preview shows up for a moment and then vanishes. The screen, which is black in Presentation mode, goes gray at that point--the whole screen.

So, not a corrupt file but something in the program itself--or my computer (2017 MacBook Pro running Mac OS 11.4). Is this a none problem, hopefully with a solution, that I've overlooked? 

Thanks.