Affects Version/s: 1.16.10
Fix Version/s: None
TL;DR: If you change your gamertag on the Microsoft site then, at least under some conditions, the iOS app keeps trying to connect to the old one and won't launch anymore.
So I ran into precisely the problem hit by joshkelsey-ross on Microsoft Community last fall:
My son has had minecraft on his ipad for a while now, and i have just got it for my iphone so that we can play together. It has been working fine until today when i created a microsoft account for him and set it up as a child account with myself as the parent. Now when he tries to load minecraft, the mojang logo comes up, it tries to load and then it closes itself.
I noticed that the app was almost loading before crashing, getting through most of the startup screen. After restarting the iPad and letting it sit for a while before attempting to launch the app again, it got all the way to the welcome screen and displayed a message something like "now logging into your Microsoft account" before the app died.
I was preparing to uninstall and reinstall the app to try to get it working at all, worried that all my son's levels would be lost in the process. So first I connected to iTunes to backup the iPad and any accessible Minecraft files in the hope I might be able to restore his games somehow if that happened.
On a whim I looked inside XBLStoage.json at the root of the file structure. I noticed that in two places his gamertag was still recorded as the default one that the Microsoft site assigned to him when I first created his account – he had just changed his gamertag before the app started crashing.
I hand-edited the two incorrect gamertags to his new gamertag and put it back on the iPad, and the iPad app resumed launching.