After some playtesting by friends, I’ve fixed a pile of bugs:

• When leaving a room, all projectiles created in that room and all monsters spawned by other monsters in that room are destroyed.
• When leaving a room, all monster behaviors are stopped.
• When loading a saved game, all game state is reset first. This prevents getting to keep a card you acquired before dying.
• Monster behaviors are more careful about their references so hopefully defeating the final boss will stop crashing the game.
• Player post-appear invincibility timer doesn’t start until player moves.
• Player is now invincible when moving through doors.
• Make sure player is always invincible when using spirit card.
• Prevent freezing player while invincible.
• Make sure player always is created with full HP on load.
• Adjust some enemies so they do not mob the player as easily.
• Replace the blobs in the cloud card room with a mini boss.