With my M1, why does Photos blow up on export when creating a slideshow with mixed photos and videos!?!

With my M1Mac Mini & latest big sur, why does Photos blow up on export when creating a slideshow with mixed photos and videos!?! The previews and play feature work great, but on export it bombs. All photos and videos come from my iPhone 11, latest ios 14. If I take out the vids, all is good. Put one in, all is bad. That rots.