I'd agree with Maergoth and Drumstixx if it wasnt a normal part of several fights in which someone is routinely charmed through NO failure on the part of any players. I know the OP is talking specifically about Avatars, but this affects more than just that.
If you put that heal on Cooldown, is it still able to go off during the charm? I feel like there are easy ways to prevent this.
the Monk's Mend ability can just be wasted every time it is up and "Should" not be able to be used by the Mob. But from what i understand, Pally's have a lot of things that reset there Lay on Hands (100% Heal) so even if it is down it can be brought up very fast.
Seen it happen on Tagrin multiple times during PoW progression. Actually just using Avatar's as an example. You are correct that this issue affects more than just Avatar fights, it's a problem that crops up in any and all fights that have Charm mechanics. It just wasn't an issue until Paladin's heals became percentage heals.