At first I figured the physical solution, not being able to climb higher than the ladder in any case, would seem to be a reasonable one. And you probably want to do that. But that might somehow end up with characters stuck on a ladder if the dismount event never fires for some reason, so making the ladder a single event and\or exempting the dismount from the "blockable furniture actions" list, should also be done.
They should probably also consider adding fall damage as part of the considerations in invulnerable mode.
Interactions should be atomic. Unexpected outcomes in a state machine should be rejected (and probably errors logged if in dev mode).
Edit (additionally):
If they can't be made atomic, make the transitions timed, with expiration, never allow deadlocks to occur and always branch to a fail-safe state. Players will tolerate things like the uncontrollable NPC getting pinched off screen and respawning, even with a comical animation if it's that kind of game; they hate a blind referee ruining things for no perceivable reason.
I don't think I agree with this if you mean climbing ladders should be atomic... what if I save/load the game while someone is in the middle of climbing one? What if the level changes somehow unloading them? What if a level designer wants someone to start on a ladder?
In the case of Outer Worlds, I don't recall having seen any ladders where it would be unsafe to pick started or finished and snap to those states over the save.
In the case of another game where someone's actually making progress up a very large structure, I can't think of any game offhand where saving in such a state has actually been allowed. However allowing a save there by necessity triggers the edited version of my statement which includes a timed state transition and safe aborts for failures in state change. Such a save would also include the current state of progression as well as the remaining limit for the timer.
The companions in The Outer Worlds do all kinds of insane things anyway. They are always getting caught on the wrong side of doors, or outside of elevators. Or you leave them behind and they just teleport in next to you.
Probably the most ludicrous is that when sneaking, only the player is taken into consideration by enemies. I've seen patrolling enemies kick my companions out of the way without actually noticing them.
All of those are mostly because any other policy would result in companion-less runs of the game. AI just isn't there yet to read and react to the situation in a way that should take such outcomes in to consideration, and arguably that effort might never be something custom tailored to a game, it'd have to be off the shelf with maybe just some tuning and/or helper hints.
Reading the player's intent would probably still be a very difficult problem, so even with the perfect knowledge of how to interact in the world they'd probably not act and react to changes in the player's intent anywhere near as well.
Fortunately they tend to not just immediately go Leeroy Jenkins as soon as an enemy somewhere in the far distance goes active.
Skyrim was/is awful about that. I'd be sneaking along through a tomb, and then Lydia screams "I am sworn to carry your burdens!" and charges in, waking up a dozen draughir.
Billions of dollars of programming time can be spent on self driving cars, not so for your NPCs.
When large amounts of programming time is spent on game ai it does end up "self driving" at an adequate level, see Google's sc2
bot, Google's older arcade game bot's, and openai's dota bot. It's just not worth it investing that amount of time into most games.
They should probably also consider adding fall damage as part of the considerations in invulnerable mode.