There was a bug similar to this posted in 2016, but unfortunately the solution does not work for me. Just this morning Krita was working fine but now for some reason or another there is a black bar underneath the brush, only when the brush is on the canvas. The solution they posted previously was to rollback nvidia drivers, but I can't do that, as the option to rollback is greyed out. I would appreciate any help as it is rather annoying. thanks.
welp! changing the cursor to "small circle" seemed to fix it (for now, at least!)
I'm sorry, but it remains an nvidia driver issue. We cannot do anything about it, I'm afraid.