Ardupilot drifts in long coordinate turn (pixhawk 2.1)

I have been testing a pixhawk 2.1 for use in GA aircraft. Today I noticed a phenomenon while performing a long coordinated turn. Initially, the pixhawk indicated the correct direction of turn and bank angle. Then after ~30 seconds while still turning, the pixhawk slowly indicates that the aircraft is leveling and then turning in the opposite direction.

Can anyone provide insight on this problem or how to fix it?

Can you provide more details?

The term “GA aircraft” is typically used for airplanes where the pilot is riding inside and controlling the flight. Of course the pixhawk is an autopilot, typically used when no pilot is onboard. So what do you mean that you’re “testing a pixhawk 2.1 for use in GA aircraft”?

Also, can you provide any dataflash logs, videos, or other demonstrations of the issue?