Plane dives when transitioning to RTL

Arduplane 3.6
Flying wing

Seems like this issue has been around for a while, but I’ve never found a cause or fix. Just built a new plane. Flies perfect - except when it transitions to RTL - as a final part of a mission or just switching from FBWA to RTL.

Mode switched to AUTO. Plane flies a short mission. Airspeed and altitude are perfect even though it’s breezy out. RTL is the final waypoint. Plane dives from 100m to 70m, then recovers to 100m. I switch back to FWBA then to RTL to test again. Same thing. Switched back and forth a few more times and get the same results.

Here’s the log: