Summery:
There seems to be an incompatibly where zombies heal shortly after being shot at. It's most likely a conflict with the mod Random Zombies - Day and Night. I noticed this only occurs at either day, or night.

Info:
I originally setup my server so zombies would have normal health at night, and tough health during the day. I noticed they'd heal at night after being shot, but ignored it at the time. Eventually, I changed the zombie stats so night zombies would have tough health, and day zombies have normal health, and they started healing only during the day. So it might have something to do with that setting.

If you're interested in fixing this, and need help replicating it, feel free to ask. I am also a mod author, so I get that not everyone has time to fix troubleshoot these edge cases.