I had one zombie become invincible/stuck in the background on the staircase of the 6th floor here. She was able to hit me while I couldn't hit her back until I got farther away from the staircase which then I was able to attack her and kill her. I thought it was weird at first but then later on I had another zombie that was chasing me get stuck on this spot on the staircase, he went up the staircase fully and walked up over to me and was able to hit me as well.
I don't know if this is bug on other locations with staircases but this is the first time I've had zombies become invincible/stuck in the background that were able to attack me but I wasn't able to attack them back.
The spot where they both died is where they got glitched as they were hitting me and I wasn't able to hit them back until I got farther away from the staircase. They were able to go towards me, attack me, and even grab me as well while they didn't take any damage from my weapon. I didn't lose any durability as the hitboxe registeration wasn't working in that spot.