I have this issue too. Running Minecraft 1.19.51 build 12894712 on iPhone 12 Mini.
Summoned the Wither and was able to run about for a bit (a few seconds) and then the game crashed. Can't re-enter world as the entire app crashes after the Generating world/Building terrain... screen.
I've tried restarting my device.
I've also set all video settings to either off or their lowest possible settings.
I've just tried with 1.19.60 too and the game did not crash when "Building world...", so this looks positive.