I found another bug, this one regarding zooming of "windowboxed" (letterboxed on all sides) .mp4 files. Changing the aspect ratio to "zoom" to make the playback fill the entire screen results in color glitches again, monochrome image partly overlaid by orange/pink. All other aspect ratio modes, including wide zoom, behave normally.
Disabling PRIME acceleration or changing the render mode from direct-to-plane to EGL works as an interim fix.