From what I understand, this is a known issue with summoners because the game logic checks to see if all team members are dead before it checks if a card should be summoned on death.
So Princess casts her spell on herself. Princess kills herself. Game sees that everyone on team is dead so it initiates end of game. Spell finishes and summons a troop. Que Win or Loss screen (you can do this to yourself, too. I did it just the other day lol).