Almost all gliders are fairly strongly spiral unstable. Because of the long wings the outer wing in a turn is moving significantly faster than the inner wing. As a result the outer wing generates more life than the inner and the bank will increase until the glider is in a spiral dive. It is normal during a steep bank in a glider to have to apply a significant roll input to prevent the bank from increasing. So not only will a glider not spontaneously recover from a spiral dive, it will put itself into one by itself.

That is why recovery from a spiral dive is something that a glider pilot usually has to do as part of a demonstration of proficiency. I get to do one every year as part of my seasonal check flights.

These high altitude, long duration UAVs have really long wings. So the control system would have to work to overcome the tendency to overbank. At some point the tendency to overbank might overcome the authority of the controls.