Have a look at this short mp4 sample video sequence:
https://filehorst.de/d/dijIsvem
I can play it with video players like VLC.
However when I load it (by drag &dr op) in Avidemux v2.7.8 in Windows its shows a mess (see attached snapshot).
Why?
problem loading in Avidemux (orig).png
Should be fixed by [demuxers/Mp4] Accept H.264 labeled as avc3 (https://github.com/mean00/avidemux2/commit/efa31757e7d0aed21cb5390fd27724e48a0efaab).