Kwiboo has given me (and thus you) some patches to test that fix the glitching/tearing issues introduced with the latest ffmpeg changes so RK356X/RK3576/RK3588 images in my test share have been updated to include them. Go forth and report issues
NB: I've also extended VP9 hardware decode (up to 1080p) to RK356X as the patch was indeed trivial. However i'm not in the same location as an RK3566 or RK3568 board at the moment, so let me know if it works?
The glitching/tearing mitigation patches appear to be working really well on the r6s rk3588s. Using the YT add-on, vp9 playback does occasionally produce a static green video output, and sometimes seeking forward/backward or restarting playback results in expected video output.