Horizon tilt changes based on what direction MP is facing?

[erkme73]

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):

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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.

 

[Kilrah]

IMU and gimbal calibration should help.
Note that even then it might not hold great when you yaw that ridiculously fast though.