Okay … that is REALLY helpful information (good to know - it explains a lot of other challenges I have faced) … thank you.
I went ahead and rebuilt the (formerly 1080p) mlt “mini project” as 4k (2160p) to match the new project(s). Et voila … that seems to have done the trick. 
… do the fps matter as well or just the resolution? (I have them both @ 30 fps right now, but it would be helpful to understand how this might turn into a new problem if had a project w a diff framerate.
But now a new problem cropped up: Since Shotcut crashed earlier, I get “missing files” when I open the project - I can remap them and save the project, but when I reopen the repaired project, it forgot and I have to map everything again … and again … and again. (why wouldn’t the new xml file store the info?)
And - just happened - when I drop new assets into the playlist, Shotcut immediately crashes … (sigh) 
(Should I revert back to the older version …?)
*edit: … uhm - even more problematic: The same appears to be true for ANY project now.
To test this, I just created a new (empty) project and tried to drag a file into the playlist … Shotcut crashes (wheels keeps spinning indefinitely w/o anything happening - only way out is to force close SC)
**update: After rebooting the system serval times, going back and forth between versions (22.10.25 & 22.12.21), and applying a Win 10 update, I finally figured out what the problem was … and it had nothing to do with any of the above.
… I had named several files with a “%” - and Shotcut doesn’t like that … most of the time (every now it then it worked - which is probably why I didn’t pick up on it earlier. 