Obviously the taxes are never going to be perfect with either schedule changes or IRROPS.

Like if I were booked on a flight that never touched German soil and then was rescheduled to go through FRA, do you really think AC would send me a bill for the German security charge? Of course not. Same reason you won’t have this refunded if your scheduled FRA stop gets rescheduled or rerouted so you don’t land in Germany.