I am also having this problem, Minecraft 1.20.12 and IOS 15.7.7 on an iPad Air 2. If I do the turning off of Wi-Fi, it runs. If I can also log out of my Xbox account, it runs. But using a realm takes some acrobatics to pull off. It seems to be whatever code connects to Xbox account that fails and crashes the system.
I am also having this problem, Minecraft 1.20.12 and IOS 15.7.7 on an iPad Air 2. If I do the turning off of Wi-Fi, it runs. If I can also log out of my Xbox account, it runs. But using a realm takes some acrobatics to pull off. It seems to be whatever code connects to Xbox account that fails and crashes the system.