Here in October 2019, I continue to experience exactly the same bug as described in the late 2018 bug reports, and in the detailed steps to reproduce given in the late January 2019 tickets. I changed the settings on an XBox child account to allow them to join multiplayer and minutes later when they attempted to log in the game began to crash on load and has continued to do so. Exactly as described in the ticket. FIX IT PLEASE.
I have not yet attempted to go through the "XBLStoage" [sic] hack to attempt to fix it, but that's a ridiculous level of fine-grained work to expect your users to do when simply using the parental controls of the XBox login in the most obvious way.
Furthermore, there is no longer a straightforward option to log out of XBox live from Minecraft from an iPad, as far as I can tell. That's a UX fail that is making this worse than it needs to be, as it circumvents some of the more straightforward workarounds that were apparently options before.
follow-up: going through that process to delete XBLStoage did work though. Just a big PITA.