On my RPi4 2GB I have experience the same problem when i upgrade from 9.2.0 -> 9.2.1. It happens to my 4K HEVC (Main 10) but not others, downgrading to 9.2.0 fixed the problem.
Am I the only one having video playback problems of 4K HEVC movies on RP4 on this build? My 4K HEVC videos have what look like compression artifacts on playback and a white horizontal line in the top. When reverting back to 9.2.0 everything is fine again.