Are you sure you have no other peripherals besides keyboard and mouse? Can't think of what else would cause this. Usually it's caused by undiagnosed joycon drift of a game pad or somesuch.
It seems that, for some reason, even having a USB pad converter hooked up causes this to happen. Was able to fix the issue by unplugging both of them, but it's a little weird given that neither of them had a pad plugged into them. In any case, looking forward to trying it out!
