Horizon tilt changes based on what direction MP is facing?

I've been pulling out my hair trying to get the horizon level. I manually adjusted the tilt, but after flying in a different direction, the tilt was off, sometimes in the opposite direction. Then it occurred to me that the tilt is affected by the direction of travel! If I face north, for example, it's level. West, it tilts low on the left. South, level. East, low on the right. To best demonstrate, watch this (keep vomit bag nearby):



So the question is, why? And more importantly, how do I solve that? FWIW, I'm in the flat lands of central FL. ZERO actual hills.