Oh! I think I see what’s going on. I was looking at linux-oem-6.14 package : Ubuntu to see which patches are in which kernel.
The VCN patch is in 
6.14.0-1007.7 which is in proposed.
The repo has 6.14.0-1006.6 as the default right now.
You need to pull that newer one from proposed for now.
here’s the detailed bug with more information: Bug #2112582 “HW accelerated video playback causes VCN timeout o...” : Bugs : linux package : Ubuntu
BTW - Besides the power management stuff I mentioned that X11 doesn’t handle well, it doesn’t handle GPU resets well, which could explain behavior if the VCN patch is missing.