Resident XML knowledge UGC gal here - there were a few ideas floating around regarding enemies being restricted not to aggro through walls during Shatters development (for a cool effect where you could actively see enemies in, say, the village patrolling w/o them noticing you), but it never ended up being developed further.
Aside from walls, “undiscovered” areas just means that you can’t see in them. In the case of Theatre, I’m not entirely sure what the exact term is, but it specifically uses a visibility setting where enemies are not “unloaded” if they’re in such an area.
Hence, Assassin puppets are able to throw bombs from other rooms, as can Sorcerer puppets summon those weird red balls o’ low-def-instakills.
The easiest “fix” would be something like changing the visibility to the more-cringe LineOfSight, which is what Manor (and Lost Halls?) used to use; if an enemy’s around a corner, then you literally can’t see them, nor can they be hit by projectiles. I don’t know if this also disables targeted Behaviors, though.
Best-case scenario is that we magically do get devwork for disabling aggro-through-walls. And then wait for an entire Theatre reconstruction to implement it, too, since it seriously use one. iirc all tiles, wall, and decor sprites, with the exception of the curtains, are just reused assets.