Seen on Randalf.
Having Arcane Affinity shows the discount in the interface, but doesn't actually apply it on pet rejuvenation or portal use.
Randalf got an Aethersprite from a blessing. It allows him to rejuvenate the pet. The rejuvenate button says 15, as if the Arcane Affinity discount applies. Trying to rejuvenate at 18 MP gives an error claiming he doesn't have enough MP. Trying again at 24 MP rejuvenates the sprite, costing 20 MP.
He saw something similar with portal use. The interface will show the discounted price, but won't allow portal use if he has MP for the discounted cost, but not enough for the regular cost. It will charge the full price to actually use the portal. Saw that a while ago, but I don't believe I ever made a thread on it.
Not sure if the bug is that the interface shows the discount if it's not supposed to apply, or that it's supposed to apply and doesn't.