Thank you so much for the comment!
There’s a possibility for the library denizens to spawn with a hostile AI, and if you’re invisible they can’t find you.
That's cool! What does that look like, given there's no health score? I'm not sure if I've seen this or not. I'm guessing you mean the ones that actively increase stress, but I could have sworn I tried using the spell around them with no noticeable difference on stress increasing; that's why I said I couldn't find a use for it.