Does anyone have any other suggestions on diagnostics or fixes?
I don’t want to globally disable GL or EGL just to make Totem work with a subset of videos. And I can’t really add it to the .desktop file for Totem because it’s got DBusActivatable=true, which means that it ignores the Exec= line.
Since those videos play with ffplay or gst-play I guess that they will play as well with showtime (now the “official” video player for Gnome) or mpv or derivatives like smplayer. Totem decode chain sometime is a mystery (at least to me 
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Unfortunately not. Showtime has its own issues (which I was going to keep to a separate thread, but got a solution for in this thread anyway, and also appears to be a GL/Wayland issue)
I don’t know why there aren’t more reports of these issues. Wayland and NVidia is presumably a quite frequent combination these days. I guess maybe most people aren’t watching old MPEG-2 and MPEG-4 videos and so don’t see it?
Well yes, most people do not watch MPEG-2 and MPEG-4 files, however all my MPEG-2 and MPEG-4 test files play perfectly fine in both totem, showtime, clapper, etc etc.
So this is in no way a universal problem, it is a combination of your hardware and drivers.
How do people not have 10+ year old video files from old devices?! 
It would be nice to work out which part of that combo is causing it. Because a 4070Ti with the pre-packaged drivers and the standard GStreamer (not Packman) shouldn’t be that uncommon.