My mistake, I should have said I bought all of the guild classes, and it hasn't reappeared in the mages guild. Just the potions/resources in all four of the guild.
I bought it earlier in my game and taught it to my player character, who still has it, but I can't give it to anyone else. Jean and a generated slave also has it, not sure if Jean comes with it but I know the generated slave I taught it to her so she'd have advanced casting.
