Yes it is bugged and easy to softlock. You can mitigate to an extent. Take a punch on purpose before shooting out-of-bounders so you have a good chance of a one-shot. Most stay upright for a second after shot before collapsing - DOUBLE TAP. There is 2 grenades on the map you can use as backup. If you "cleared" the map but apparently one of these hard cases survived, they will be moaning still. Grenade them if you have one, if you don't, restart the map.However, do any of you have a problem with beating Mission 4 (originally Mission 3)? I'm hearing it's bugged, but not sure if y'all have the same problem I do - mine is that one or more of the zombies in the green carts are unkillable; you can shoot them when down, but they don't vanish like the others, yet they groan like living zombies. I'm certain they're the only ones on the map preventing me from beating the level since the loudness of the moans while next to them means they're surely the only living things on the map. I also used the free-cam off UU, and I can't find any surviving zombies otherwise.