Not much to explain, the only way to get rid of them seems to be the debug kill all monsters, they can have c4 blow up at their feet or be crit with an awl pike and remain uninjured. The version I was running before was 5241 and it seemed fine there. Newly spawning Shambling Zombies definitely are also invincible.
Wait, no. Strange. I went back to my 5241 backup, killed one Shambling Zombie just fine but found another that was also invincible, so perhaps it is random or there is something more I’m not seeing.
Edit:
While trying to reproduce the problem after reinstalling 5241 again, came across a Big Blob that at first seemed to be invincible, taking no apparent damage, around the 10th hit or so it just suddenly died and split, not sure why. Haven’t found another Shambling Zombie yet. Except one I spawned in with debug and it took damage normally.
More Edit:
Still 5241.
Alright, found one, and looked at the fight a bit closer. At first it seemed to be invincible as well, but after about more than ten minutes of stabbing it (the log doesn’t go past that) I noticed it was at yellow health, and one more hit ended it. So it almost seems like I was hitting the arms and legs, uselessly, and inflicting damage that is irrelevant to zombies. This time I made a save right before engaging it and tucked that away in another file, so I can replay the encounter easily.
I know it’s the sort of things that can happen to players as long as the arms/legs are hit, but it’s not supposed to happen to zombies, is it?