I recall from research at "the Krib" that iron lasts a very short time in an aquarium, before the plants take it up. As I recall, people were dosing iron, then measuring how much was in the water and getting near zero measurements. The "expert" opinion was that the plants didn't allow it to remain in the water more than minutes before it was consumed.
If that was correct, or at least if my memory is correct, that tells me that activated charcoal in a filter doesn't get much opportunity to remove chelated iron from the water before it is used up. The other chelated metals in trace mixes may linger longer, and be removed by the AC.
Is this correct?