Unknown RTL after TakeOff

Using Pixhawk4 and Arducopter V3.6.3, starting flight with ALTHOLD mode, copter switches to RTL at 2-3m altitude, with circular GeoFence (60m altitude, 120m radius) active. I have experienced this behaviour twice. The problem went away after disabling GeoFence. I have checked all other options that can trigger RTL, but everything seems fine.

Did anyone experience this problem?

Does it have a good 3D GPS fix before taking off?

Posting a .bin file from the Pixhawk would certainly help any analysis.