ToME: the Tales of Maj'Eyal

As statet in the title I noticed that the Venomous Ammunition of the 'Exotic Munition' skill poisons skeletons, which are 100% poison resistant. (see attached screenshot). I guess the effect somehow doesn't count as poison and is there simply applied to everything.

I am using the steam ToME and am therefore up to date on versions (1.5.5) and am using the add-ons ashes-urhrog, orc, item-vault, possessors and better_item_desc.

The skeletons I noticed this on were in the ruined Halfling complex, if that matters somehow.