I have found that some class specific items seem to be bugged for certain characters and or classes.
All Mage only items are not recognizing ARAINE as a Mage although she has that class trait.
Similarly The Shield Of The Sentinel will not work on any Defenders (Sovereign or otherwise).
EDIT: Sorry I forgot to note that Mage only items do not work on Sir Kodrin either.
It is looking like any Characters you receive from completing quests cannot equip class specific items.
EDIT: The Sovereign I created cannot equip the Sharp Longsword (Assassin only) even though he is an Assassin.