I asked for your OS because Windows 7 has some extra stuff that messes with tablets, but I don't believe they exist on Windows 10.
I'm willing to bet this issue occurs outside osu! too, although it's probably a bit more difficult to notice with your Windows cursor.
I'm not really sure if you can do anything it that other than by disabling the tip of your pen if disabling Windows ink isn't work. Since I don't believe your drivers allow you to really do that, try checking in the Pen and Touch settings in Windows and try disabling it there. You might want to consider just taping it down or something if that doesn't work.