I recently acquired the Stinking Mask and picked up the Mestil's Acid Breath spell to play around with it, on my wizard.
It does not do the special "//This item, when equipped, increases the DC of Mestil's Acid Breath by 2 and gives a 10% acid vulnerability to the targeted enemies"
My DC was still 22 when I got GSf conjuration, 5 int mod, and it's a 3rd level spell, so 10+4+5+3 = 22, and there was no acid vulnerability when I hit stuff, tried it on my own summon and tried it on another player.
If it helps, I believe this item typically spawns in troglodyte quests? Possible Lizardfolk ones too.
There were a few items like that last chapter that enhanced some spells, but it wasn't carried over.
Just ignore it and we'll remove the comment in the desc of the items.