There is a Feat in 5e called Sentinel which does:
Whenever you hit a creature with an opportunity attack, its speed drops to 0 for the rest of the turn. This stops any movement they may have been taking.
• Creatures within your reach provoke opportunity attacks even if they took the Disengage action.
• When a creature within your reach makes an attack against a target other than you (and that target doesn't have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
That feat is currently not in the game yet for us to choose, but it almost feels like what is happening is this.
However even if that is happening it shouldnt be consuming your action, just your movement.
If its not that its prob a bug.
With that said I have also found mobs that have unique mechanics to trigger AOO and learned quickly how to adjust. Its possible they gave special features to certain creatures.
Jaz
It's definitely not an intended feature. It happens to me often against any enemy. They aren't using polearm master, sentinel, riposte, or any other source of an AoO that should proc.
