This has been happening since early betas in an unpredictable fashion, and it is super disruptive, but I didn't have a good reliable repro until now.
AVF sometimes renders video as a random solid color.
Repro: I have the same footage encoded a MJPG and HAP. Put both in modul8, in two layers, move them left and right so I can watch both at the same time. In the media info tab, verify that MJPG version defaulted to FFMPEG decoder, and HAP is only supported by AVF. Switch the MJPEG clip to use AVF decoder. The hap clip turns to a solid brown.
Obviously this should never happen, and it is extra odd that decoding of different clips in different layers is not hermetic. The fact that different clips can impact eachother through the decoding library means I have some projects that are very unstable; making simple changes (swap a media in a layer) sometimes totally breaks something somewhere else or later in the performance.