Thanks for the report! I checked your account and the drone was bought, but it was never equipped on a tree. Buying it only adds the charge to your inventory, and it only works on trees you arm it on. Your charge is still there, nothing was lost.

How to use it: water the tree first, then, while it's on cooldown, tap the drone icon on that tree to arm it. From then on the drone waters it automatically a few minutes after it's ready, even while you're offline. We also added a hint after buying a drone so this is clearer.