Sadly, with no-media-buffers it got much worse, to the point that actually nothing plays normally on my 1 IV. It’s not just semi-transparent bars but the entire image looking like if it was twisted / scrambled or something, nothing is visible.
Example screenshots:
So for now I’m back to video/avc=0 
Does it work OK on the 5 IV? If so, maybe 1 IV’s different display resolution is responsible for that?
P.S. @remote’s sample video doesn’t play in Gallery no matter if avc decoder is enabled or not (it plays just fine in web browser, though). It actually makes the Gallery freeze, without showing a single frame. But that’s actually the only video that doesn’t play at all, no matter if the hw decoder is on or off. With hw decoder disabled most other videos I ever encountered play fine, and only around 5-10% have some bars or stripes of varying intensity, something like this:
On the other hand, if I enable hw decoder and no-media-buffers, everything gets totally scrambled as on the three screenshots earlier. Removing no-media-buffers makes most videos look entirely correct, but after a few seconds videos stutter and then freeze 