MP is not working when the drone has an ADSB receiver (specifically the PingRX ADS-B). Every time I hit the connect button, the MP attempts to connect without success and the signal strength is always around 40% or less. When I unplug the ADSB receiver, the MP successfully connects to the drone every time and all works just fine.
I think this is a bug in the MP
Set the do not forward mavlink by option on the serial port that you connected your PingRX. It sends heartbeats with wrong parameters and it confuses Mission Planner.
I think the root cause is that PingRX sends out heartbeats with ID1 and component 0, causing out if seq errors on the MP side. (At least that was my experience). MP does not disconnects once connected, but it makes connecting very slow.