Trasmogrant's crown has two equip costs and it looks like Arena handles assuming if the Black mana can't be paid, to ignore checking for the 2 generic.

Normally, that's logical. But Firion and warwhip in standard both can reduce the cost of the generic equip to zero. But Arena will skip through phases assuming you cannot do anything.

You can work around this with full-control, but it's a pain.

To recreate, reduce equip cost to zero and spend all mana. Play will pass to the opponent as if you can't do anything. And the crown will not gain a highlight boarder, like when a card is usable.

It's noticeable with 2 warwhips. With one in play, the crown still highlights and offers you the option to choose from the equip options. But the second warwhip comes down and the crown's highlight turns off. You can still use it and choose the 2 generic option, and it'll be free. But if you tap all your mana, play will progress automatically.

Included a log while playing it against a bot and encountering the bug at the end.