I've just had the same problem that I had once before
Loitering normally, throttle half way, everything all good. I then click disconnect in the Mission planner, all fine, but then reconnect and the aircraft instantly falls out of the sky.
This has happened 3 times to me now, most of the time its fine disconnecting and reconnecting, so its an inconsistent issue.
Could this be an issue with the atmega on the apm being overloaded, do i need to move up to a pixhawk type FC?
APM 2.5 running 3.0.1 with 1W ultimate LRS
Log is attached.