The more I look over how Attack of Opportunity works in this system, the more I feel like it starts having unforeseen issues with monsters that have it just because of how many actions wind up interacting with it. One big source of trouble is the number of actions and feats that, thanks to using certain "subactions" as part of them, bring in a manipulate tag that makes it more dangerous/subject to interruption than it probably should be. Magus Spellstrike was the first thing to bring the issue to my attention with its Cast a Spell subaction, but while I've seen plenty of discussion with good points both ways on that being intentional, things get a lot crazier when you bring all the feats that allow Interact actions as part of them that suddenly become terrible ideas if you happen to face an enemy with AoO. Quick Draw any sort of melee weapon? Your iaijutsu samurai who draws his blade fast as thought... will always eat a blade to the face first from the level 1 orc warrior. Rogue with Mug, who can cleverly lift an item when they attack? Get pounded by an enemy who doesn't even know WHY they did that if you succeed on the Steal check. And Reload is an Interact, pretty good argument by RAW Reload 0 would still have that manipulate trait even if it doesn't cost an action. Mobile Shot Stance becomes kind of pointless against AoO monsters if the ranged Strikes don't eat an AoO but every reload does. Most of the magic items at least seem smart about using Command instead of Interact but there are a couple of screwy exceptions like Dagger of Venom (free action to poison? Not so free now!). And it kind of sabotages groups who don't want to rely on healbot casters since Battle Medicine and champion Lay on Hands suddenly have issues once AoO enters the scene.
Worsening the issue is how many monsters have additional traits that compound the danger Attack of Opportunity brings. A large percentage of them have it with Reach attacks; you can ask all the fighters adopted by gnomes and trained in the flickmace just how valuable Reach can be for AoO. A fair number with 15 foot Reach to boot which gets pretty obnoxious with approaching them. The fact it's frequently "boss monsters" who have it causes problems because they're most likely fielded as above-level enemies for a party, so the increased likelihood of a crit on AoO increases the chance of a PC wasting actions. Don't get me started on the ones with "improved" versions of Attack of Opportunity. The monsters who can do multiple reactions for it can be bad enough, but the few who can interrupt on hit instead of crit can really mess up the action economy for PCs. My personal favorite, Mummy Pharaoh, who interrupts on hit AND can do it against concentrate actions, is so utterly, utterly broken. Want to activate ANY magic item, Hunt Prey, Devise a Stratagem, command an animal companion, Demoralize, or use a monk ki spell (or ANY spell even verbal only)? Get dunked on! Recall Knowledge? "I just remembered this is a bad idea!" *SMACK* "Damn it, what was I saying?!". Reach 10ft and mummy rot to boot. About the only limit is they at least don't have the "make multiple AoO" ability, otherwise everybody would stay the Abyss out of Osirion if they know what's good for them.
Now thankfully, especially with more recently released monsters, there seems to be some recognition of how AoO can have unintentional effects because I see a lot more specific reactions that only come up for particular actions and generally if they allow attacks don't allow interruption of actions too. But it still pops up often enough that it might not be a bad idea to make a few changes to prevent screwball interactions. At minimum I'm thinking maybe we need a blanket rule that any action that covers subactions should either have the traits of those subactions listed as part of it and/or be treated as overriding any traits of those subactions with its own. I have a sneaking suspicion part of the reason we have actions with unexpected vulnerability to AoO is the writers can't always remember which traits are included in those subactions, so this rule would prevent oopsies and confusion down the road. It would also help a bunch at the table, because I guarantee no group is remembering every single thing that should be subject to AoO, intended vulnerability or not, and this would at least make figuring that out easier if not perfect. I also think Battle Medicine and Lay on Hands should have some changes (or maybe feats at least, though they might become a feat tax) to support using them in an AoO environment, because I think moving group composition away from always needing a heal caster is a good change in Pathfinder 2E that should be encouraged, not undercut. Maybe Battle Medicine could get no AoO or at least increased AC against one at higher levels of Medicine skill?
Attack of Opportunity is one of those things that has a lot more twists to it than most people consider, because of how it can mess with Pathfinder 2E's fundamental action economy (not just extra attacks, but a chance to force an enemy to waste a precious action). And since PCs are far more likely than monsters to have actions beyond move/ranged attack/cast a spell that trigger it, use of it on monsters needs some consideration because it can get twisty surprisingly fast.