According to Arch Wiki

https://wiki.archlinux.org/title/Hardware_video_acceleration

you need

MPEG-4 Part 2 is disabled by default due to VAAPI limitations. Set the environment variable VAAPI_MPEG4_ENABLED=true to try to use it anyway.

I tried with cingg/ffmpeg7 and it seemed to work, but ffmpeg7 had all our patches including patch8

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