Thanks for reporting this, you found a real bug! When two of your trees with the same reward became ready at the same moment, the drone paid the first one, but the second payment got rejected by the blockchain as a "duplicate". That tree then stayed waiting until you watered it yourself. You didn't lose anything: you got paid when you watered it, and no drone charge was used on it.

It's fixed and goes live with the next update. Also, from now on, while a drone is armed it's the only one that waters that tree. It runs every few minutes, so the tree may show as ready for a short while before the drone gets to it. If you'd rather water by hand, just unequip the drone.