MKVs yes, I saw a lot of issues with VAAPI and FEL thanks to this issue report : https://github.com/CroqueMr/libreelec-x86-DV/issues/29 , I think I already fixed it in my internal build, I need to clean my code before pushing an update of the debug version.
The issue is wider than MKV, it impacts also MP4, M2TS, everything has been fixed.
According to the issue report, the person cannot playback DT DV. I have the DT test file of "This Device Can Play DV" but for the DT test file specifically, it only displays black screen on my AM6B+ on CoreElec NG AND on your builds same thing. I thought this was normal and unsupported by ffmpeg or something. If you fixed that too that's pretty neat.