Cerulean Bell forces 1 card to always be selected.
Upon using death to copy the boss’s selected card, the copied card will also always be selected, creating 2 cards you can’t unselect.
I’m not sure if this is intended or not, one way you can look at it is the death card copying the “always selected” state, but personally I don’t think that’s how this interaction should work and I’m assuming it’s just an oversight in the code.