Notable Points:

Although I have joined from both computer and mobile device, at the time of this glitch I was rejoining on my mobile device.

This was on a Bedrock 10 player realms, that I have played on for a few years without issue.

The photo posted is of where I was teleported to. Exact same position, but I placed protective deep slate around my character.

Description: 

Background context: I had recently made a sculk farm underneath a zombie spawner. Because of this, I was logging in and sitting afk while it would run on my phone often to let the sculk build up. Anyway, my coordinates were something like 1500, -10, -2000 somewhere. I don't remember it exactly, but it was something like that. Anyway, I decide to log on for probably the 12th time on my phone today. This is when the glitch occurs.

Immediately upon logging in, rather than my character appearing wherever I last was, I was suffocating. Upon mining the deepslate block, I discover myself in an unlit cave. I am unsure of the cause or reason, or what provoked this glitch, but most notably my Z coordinate got suspiciously set to 0, of which it was not previously. I'm assuming some glitch occurred during logon or logoff that caused it to lose my coordinate location on the z axis for whatever reason, and it got set to 0. 

Steps To Reproduce

I'm not quite sure how it happened, but presumably logging in and out of a realms multiple times on an IOS may reproduce it. Maybe I accidentally terminated the application, or moved away from it without proper logout or shutdown procedures.