I've encountered this bug generally where cards you didn't meet the requirements for would highlight after playing a card that had no effect on you meeting or not meeting the requirements. For example:

Ozan's snake card that removes 3 health from him and 5 health from rival. I started the chapter with 4 gold. Death Lotus Assassin was greyed out until I played the snake. The snake gives me no gold though so still I would not meet the requirements but Death Lotus Assassin became usable and highlighted regardless.