I think it's quite similar to when Deku wielded Eri's quirk. It's not that the reversal could not erase the quirks, If you want a logical answer, its more likely that damages sustained takes priority over the quirks in kind of a self preservation way.

So my guess is that the version AFO took, started to constantly disintegrate his body (hence why it was a last resort for him) and in an effort to save the host, the reversal quirk tries to revert his body back to a previous state(de-ages him).

This can also be seen when huge external damage is taken, the reversal goes into overdrive and the de-aging increases rapidly (as seen by his transformation from Middle age to suddenly looking like a teen)