I'd be curious to know. If you can't find anything I'll double check this one.

well, i greatly appreciate the feedback. i've seen others with the same bug talking about it on the base mod's comments, but the author never really responded.
I'll have to try disabling mods like you suggested, there must be a fairly common mod that lots of people have that is causing the issue

I just loaded the game with the base mod and had no immunity to acid with my group of shek. Looking through the mod changes, I don't see any obvious sources for you problem, such as race changes.
I'd suggest loading your game with only one mod at a time to pinpoint which is causing this. You can save your current mod order by backing up your mods.cfg file in Kenshi's data folder.

does this fix the known acid bug in the base Mando mod?