@sar104 not sure I agree with your 'conflicting signals' hypothesis above. I'm thinking the IMU's principle function is maintaining aircraft stability e.g. not allowing pitch/roll/yaw movements beyond limits ... the aircraft knows what pitch/roll/yaw it is at and how fast that pitch/roll/yaw is changing (accelerometer triad, angular momentum triad) and when limits are reached, the prop pitch is 'dialed back' accordingly, which is why you can't roll or invert a Mavic. I guess it could also be used for micro adjustments to counter wind gusts, pressure drops etc. I'm really not sure it has anything to say about position or heading, computed or otherwise . Now the compass, yes definitely, as you say GPS only informs where you are, not your heading (except when moving, as you say). I would argue loosing either compass lock or GPS lock would warrant ATTI mode, and my hypothesis is that the IMU has no part to play in it.
Sorry but your hypothesis is wrong. SaR is spot on.