The problem isnt with the immunity but with the spitting acid ability. Even though one might think that it is poision (green debuff icon and taken dmg animation) it doesnt say anything about poision dmg in the description.

That doesnt mean that it is neccesarily wad because afaik it is the only thing ingame that has no specified kind of dmg with the result that there is no possible counter to it (cant douge or resist spell/skill, dmg is not reduced by any kind of armor or resistance). (Immunity to magic should block it like every other ability but that is a confirmed bug/design error caused by handling abilities like spells.)

I'm still doing some work on this (because it's hard to setup) but I still think poison resistance is bugged.  that's because the game treats poison that is applied directly from a Weapon (such as Venemous Shortbow) and posion that is applied via spell-like ability (such as using a vial of poison) differently.