I found out why.The game registers Mystic Box's effect as your opponent's when you take control of the monster you gave them and then let them have it back. It happened when I used Mystic Box to give them a monster and then used Jowls of Dark Demise's flip effect to get my monster back and then just ended my turn. My opponent then got their monster back until I blew it up with Trojan Blast
For people who want to try to use it, you can't use it against Reliquished because your monsters wouldn't count as monsters anymore but as equip spells. You also can't use it against magical arm shield because it will force you into the damage step and Trojan Blast wouldn't work. It can work against flip effect cards like Jowls of Dark Demise but your opponent has to flip them first as it wouldn't work if you attack into them.