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.
In D&D, you can choose to not take a reaction. You might let a goblin run away from you if you, for instance, had readied an action to attack the enemy wizard as they start to cast a spell.
As it is now, you can turn off opportunity attacks, but it's not a toggle per instance. You've either got to open up reactions as a limited resource or let us decide when to do them.
