Artifacts, but only in some mediaplayers

I was randomly going through some old stuff and noticed artifacts in an animation, that weren’t there before.

Top left: Potplayer
Bottom left: Media Player Classic
Right: DJV Viewer

It’s the same MP4 file. Not different versions.

I posted the animation here back then and it didn’t have the artifacts. I also just uploaded to Youtube, to see how it would look there. No artifact there as well. I checked the file in Nuke and no issue there as well.

I’m wondering why I’m getting these artifacts and only on some players: broken outlines (top portion). Very pixelated bright splotch. Also dark splotches but they are harder to make out here.

The bright splotch is only there for about a second. The overall quallity is lower in Media Player Classic and DJV View. It’s as if the compression is so high, that there is banding - like when fading from or to black and it fades in steps.

I checked player settings and don’t have filters enabled. DJV Viewer is mainly for caching and previewing image sequences, and have even less video options.

There is no alpha channel. I don’t think I have the original scene. It was a test/debug thing I was looking at and the scene had as far I can remember, it had uniform background, the object and a point light.