Occasionally I am getting some random lines in Painter 2021. This looks like the same issue I was having with Painter 2020, just much less frequent.
I am using the current driver on both my Cintiq Pro 24 and My Mobile Studio Pro 16
WacomTablet_6.3.20-2 I've gone back as far as WacomTablet_6.3.31-4 but there seems to be a limit to how useful this might be since the driver updates sometimes are needed for new versions of Windows, of which there has been a recent flurry of updates.
The Mobile studio pro doesn't exhibit the problem. It seems that only the more powerful computer has issues. This may actually be a property of the speed of the computer. As an engineer I have seen problems with real-tiime inputs mis-behaving because the hardware has become too fast for the way the input is processed...just sayin'...