Agatha pulling a fast one on everybody by starting the fight with her powers already unleashed. How else is "mystery magic" going to work other than plot twists and intrigue?
Bonnie's theft magic has most likely more requirements than "I steal ur heart lol". Mona Lisa just happened to be too shocked to work around it from the consecutive physical and mental distractions. Other witches will probably keep their distance and stay moving so that Bonnie cannot focus her ability, unlike the "static" chest of weapons she has in her room. (Plus points for her opponent moving the chest just before the next fight)