AP 3.1.1 Yaw Bug?

I have an FPV UAV setup with original Pixhawk with APM:Plane 3.1.1 and Drotek NEO-M8N module with compass and RFD900p telemetry. I had a flight when I almost lost my plane. It had to be a long “there-and-back again” autonomous mission. Weather was warm and calm with wind less than 3 m/s. Not long after successfull start with bungee at near 1500 meters from home point I noted that yaw indicator on Mission Planner (1.3.26) map begins to drift significantly more than side wind can turn it. I gave a RTL comand from Mission Planner and than behaviour of plane becomes unprdictable. It started to hop randomly around the map. GPS looked OK with near 20 satelites. I had FPV setup and saw that plane stil kept stabilized and held altitude well. So it’s became obvious that it’s a navigation error. After some struggle with FPV zoom (camera stuck zoom on scratched canopy) and trying switching between FBWA, RTL, AUTO to resume control over plane it went into direct sight and was able to land in manual.

I had few successfull flight with this plane and same parameters before this flight, some flights were about 1 hour in air with windy weather up to 10m/s wind, so it was surprisingly for me. When I look over log from flash, I saw that plane had consistent track, but had big difference between ATT_YAW and AHR2_YAW. What was the reason for that? I had AHRS_USE_GPS Enable. Is this a reason? I see some compass glitches but EKF filtered it well.

What was the reason for unpredictable behaviour of autopilot? Or it was just error of reported to GCS telemetry and plane went to waypoint correctly?
[attachment=0]ArduPlane 3.1.1 Bug.jpg[/attachment]
Telemetry Log
DataFlash Log

Yaw graphs AHR2 vs ATT
[attachment=0]ArduPlane 3.1.1 Bug Yaw Graph.jpg[/attachment]