hi. I also had similar problems with ac but I was using version 3.6.9. I think problem is in Pixhawk 4 hardware. Problem seems to occur after updating AC and import of old parameter set. If I manually create parameters by hand, problem went away. That had happened two times but it occurs, yes randomly.
Problem looks like this in video I did end of last year. This particular event went away by updating missionplanner ?!
So whole phenomenom is really random. I will try to find log files to event for that video today.
Here is topic of crash story with logfiles which destroyed drone pretty bad.