It's been happening for a very long time and I usually just go back in the history, save and open again, but this time it bugged me a bit so I decided to post it as a bug:
It happens at random, sometime I can go without it happening for several months and then it happens again, with completely random files.
Basically you can look at the video and you'll see the mask starts to degrade as ifi it was corrupted file or something similar.
For the amount of time I have seen it happening, I have had 4 computers, both laptops and desktops, countless different Photoshop versions (I try to always be up to date)
Just to state that there will be no point sending a file to someone else to check, as saving it at a state where the mask is correct and open it again solves the issue. If I save at a corrupted state, Photoshop saves the corrupted state and one cannot recover the original state.
I never save the corrupted state as it's client's work, and it didn't happen today so I couldn't share a .psd here, but I will share in a comment if it happens again (and the files are shareable)
To reiterate: it's absolutely random (I honestly think it's been happening for at least 10 years, but difficult to be exact on this)
It never happens with a vector mask.
My GPUs have always been compatible.
I am not too bothered as I have been living with it for quite a long time, but just in case it can get resolved....