So this is an issue with almost all UAVs I've tested including the vanilla Arma 3 assets. However was hoping you could extend the range of the Watchkeeper so that the fuel would last a full 4 or so hours to work around the bug.
The bug itself is that to stop the UAV trying to take off by itself after use, you must use the "land" waypoint and allow it to taxi past the exact point you marked on the UAV Terminal before you can "Engine Off" and it not try to take off by itself again.
However, it appears once you have used the "land" waypoint you are unable to use any further waypoints and after take off the UAV will just continue in the same heading.
This makes landing and refuelling the Watchkeeper (or any other UAV) unfeasible as it then becomes unusable.
As stated above are you able to introduce in the next update a vastly extended range for all UAVs you release so this problem can be avoided.