I have more info. If I view the party as a whole (using the button in the top right), Kyra and Merisiel are DUPED (see screenshot). So there's 4 total characters in the party (2 each of Kyra and Merisiel) which explains the weird numbers in the bottom. I cannot see any way to change the decks for the two phantom characters, so they have too many cards in their respective decks. This is what I assume is blocking me from advancing.