When using a Filled Fuel Can to power a Generator, the can isn't used and stays in the slot.
It initially should put an empty can in the slot replacing the full one.

@Erik3003 does the Generator run?
If Nope then it is a bug.
Else that is new Feature it uses not all of the Fuel from the FuelCan so you can be more efficent at furnaces and have simply more fuel with you and do not use everything up^^"

Ok, it isn't a bug :D
Could you please add a damage bar to show its fuel level?

added...