Just to address an issue I've seen every time I face this level. The monsters at the spot shown in this screenshot seem unaware of my presence and do not engage. They aren't even bothered with me if I shoot them, they simply sit still until they die. Similar case here but a bit further in the level.
A somewhat similar thing I have noticed when a monster gets stuck running against a wall around the corner with the purpose of trying to reach me I'm guessing. The monster keeps running against the wall unless I hit it.
Bugged or somewhat broken monster AI here?
Edit: Just to note, these screenshots are for temporary use only to point out my issue in this topic.
I wouldn't say broken really, just not the best. Though at such a distance with their backs turned they wouldn't really be expected to notice you. I like it better than those games where every enemy everywhere is immediately aware of your exact position and instantly moves to attack you.
As with many games, monsters here seem to have a certain wake radius outside of which they don't engage. I would agree that if they take hits from you then they should probably engage at that point, though.