I'm not sure why AoO isn't working properly, as it works perfectly fine in DoS /shrug. It's just a coding glitch that needs fixing. The attack should take place on the 1st enemy that provokes it - also not sure why you should be able to choose when to use it, as that is just slowing down combat even more for no apparent reason.
That's not how the rules are written, and depriving the player of the choice eliminates some tactical flexibility.
As mentioned elsewhere in the thread, reactions are a resource to be managed. You don't always want to use them.
Moreover, always using them would makes the choice of whether to Disengage before moving way more powerful. Right now, if I step away from an enemy without disengaging I can force him to consume his reaction, which means the next character up can move without consequence.
