This is an unusual freeze that only occurs with the planechasing event, any version of it.
If you win a match that results in unlocking a card from the vault, a popup comes up that will take you to the vault. This popup occurs before the event rewards are awarded. If you click on that popup, and go to the vault, you have left the event screen. If you go back to the event screen, pick planechase, quickly hit the node to play another event, and quickly hit fight, you can start a fight before the events rewards are awarded. When you do this, the fight you just started is completely unresponsive to touch, though the make a match hint activates. The only way out is to force close the app.
This can easily be avoided by just slowing down enough to let the rewards flash across the screen, but Ive triggered enough times to report it.
Ive tried doing this with other events, but the furthest I can get is the pick your planeswalker/click fight button screen before the rewards pop up from the previous fight.