I'll do a follow-up to my last post....
Seems the problem was being caused by Google Photos. After adding a few more pics (& still not showing up in Photos...) I checked to see if they were there in the google app. There was a thumbnail of ONE of the pics with a spinning icon as if it was busy...it never altered/updated!
I restarted the tablet again and some appeared in Photos (but not ALL of the ones I’d selected). I went into the Google app and disabled sync, selected a load more from the Files app, and then they all appeared
I’ve left off sync in Google Photos for now and will turn on manually to refresh with any new pics I’ve taken on my Android phone....