Same issue for me on Windows client.
Instead of entering the deck edit screen, go to the deck details from the list. After changing the format there, enter the deck edit screen and restart the Arena. By selecting “Resume Editing” to the prompt “Resume Editing Deck?”, the format change was successfully applied.
Same issue for me on Windows client.
Instead of entering the deck edit screen, go to the deck details from the list. After changing the format there, enter the deck edit screen and restart the Arena. By selecting “Resume Editing” to the prompt “Resume Editing Deck?”, the format change was successfully applied.