I tried casting this, drinking the potion and using a charge from an item and none give the +4 Fortitude save that you should get. In fact it does nothing.
Its only supposed to be +4 fortitude versus Poison, are you sure you tested it against poison? As I'm pretty sure on all past characters I've had, it works properly against poison.
It has always used correctly when I have used it.
Indeed, it's only vs poison, so it's easy to mix it up.
My newb-like knowledge of mage spells shines through. Apologises yes thats fine just tested and yes the poison only bonus is working fine.