The drone 'Jimbeam' requires 15 energy to fire, dealing 10 energy drain initially.
If the opponent has an energy shield, the shield will absorb some damage for energy an energy loss. For example,
Jimbeam hits for 31 damage, the opponent has a level 20 energy shield, and 45 energy.

31/3 = 10.3 > 10, shield absorbs 1 damage for 1 energy = 10 energy cost, + 10 energy damage.
The opponent will lose the initial 10 energy damage, + another 10 for the shield, that is why it can hit for more.

Was your mech cooling down on that turn? Were you unable to take an action because of that?
If the answer is yes, than it's not a bug, if you get overheated enough to be forced to spend your whole turn to cool your mech down, the drone doesn't fire, even if you have enough energy (or whatever it uses). But it can still fire if you can take an action, even if you use it to cool your mech down...
If the answer is no, I don't have much to say about that,
Cheers