Typically undead mobs cannot be afflicted with poison, and that includes all variations of zombies, but the Undead mobs are able to be poisoned, and I feel like they shouldn't.

1.21.4 on fabric 0.16.12 mod version 1.1.1 but I'm assuming this is the case across all/most versions

Thanks for reporting, I can confirm this bug. Seems to be a problem with the mod for 1.21.x (1.20.x seem to be fine).
I will work on a fix.

I found the problem will be fixed in the next release.

I'm currently starting to roll out release 1.1.2, that should fix the issue

@ATrueGoldenFish did you have a look?

I assume this is fixed now