All of the military GPS receivers also use the civilian signal to initialize their position to lock on to the military encrypted signal. I would assume that in the event that they lose the P code, that they would revert to the C/A code.
If a human being lost GPS signal, they'd start using landmarks, or based on their last known position, just use dead reckoning to get over friendly airspace, then ask for help. Programming that into a drone takes time and so costs money. Maybe the drone was the victim of cost cutting?
