Another issue with OA.. Charmed by the Harpies on the beach, and made party members that were in melee with other Harpies move away from their targets to go to the one charming them. This triggered OA each time. 5E rules would not grant OA when the subject is moving out of their control, such as a Push/Shove or being Charmed.
Actually, you only don't provoke an opportunity attack when something moves you
without using your movement. While charmed by the harpy, you DO use your own movement and thus should provoke AoOs. For the same reason, a creature affected by Fear and forced to run away from the source of their fear still does provoke AoOs.
Additionally, the harpy's "Luring song ability" specifically states that you still provoke opportunity attacks, at least according to D&D 5e.
While charmed by the harpy...the target must move on its own turn toward the harpy by the most direct route, trying to get within 5 feet. It doesn't avoid opportunity attacks, but...whenever it takes damage from a source other than the harpy, the target can repeat the saving throw.
An important part of the above quote is the last sentence: taking damage allows an immediate saving throw to end the charm effect. Idk if this is what happens in BG3, but if not then the harpies have been given a pretty significant power boost.
