Yeah it is /was bugged. Sorry about that. I'm glad you had fun anyway.
Priests getting exp for healing even at full HP is something I know I need to disable, but I don't think it harms the game very much so I haven't done it.
Closing the levelup page after a while is a relic of back when it just stayed open for a little while instead of being manually closed. I'll make sure to keep it open until the player deliberately closes it in the future.
Edit: Hang on, the priest retaliated? Wew, I really need to fix that then. Healing spells are technically weapons so I guess that allows the code to run that makes them counterattack. Funny.