I am currently playing Battlegrounds with Tamsin Shadow Mage. When I used my hero power “Fragrant Phylactery”, I had 3 minions, and it ate all 3. For the rest of the game, it kept destroying 3 minions. I think it has something to do with using the hero power with 0 minions on the board. I did this two times before I began putting minions on the board.