Hello there,
I recently downloaded the Affinity Photo trial for Windows 10 to test the thing so I'm still learing about the program. My problem is basically, after a while all of my brushes ( custom and standard brushes) started to behave in a weird way. All of them were very pixelated and binary color wise. I was able to solve the problem by restarting the program and the brushes went back to normal. However, the next day the same thing occured again so I'm kind of confused. I started to test things out and the only thing I found out was that it doesn't really matter if it's a new or an old document, so at least for me it happens on both.
I do work with a grafic tablet that's a bit older, but the issue occured without it as well, so right now I'm doubting it's the source of my problems ( but it's not impossible)