Hi Duke – thanks for the suggestion, but when I right clicked, “Set Poster Frame” was grayed out, so wasn’t able to do it.
I’m starting over with this particular project, in CC 2017, from the very beginning of re-ingesting the source footage, and rebuilding my timeline in the same fashion, then exporting after just a minute of edit, but making sure each clip or angle has been utilized – so far so good – then I’m going to recheck after adding filters, and see if that changes anything…I probably should have added to my original post that in my original sequence.
Hopefully the problem won’t recur – but if it does, maybe I’ll find the trigger early on.