I have yet to uncurse an Item that requires me to tie up a mage to use their mana. Either it doesn't work or it should be clearer how it works.
Just to be clear, I have to capture them of the map? Because that seemed counterintuitive to me, Considering you unlock a lot of stuff by being near it and you lose your chance to unlock the cursed Items that require the shrine to a goddess. So I thought I just needed to be near a tied up Witch. Also IIRC it says "A mage" not "a couple mages."
Mayhaps it isn't a bug, but the wording may need some readjustment.