What I did :

• create a 3x3 pool of lava above the ocean (ocean and lava part is important)

• a few air blocks above this create a 3x3 glass or leaves platform

• logout on the glass/leaves platform

• log back in under bedrock (you get "fall out of the world" death) or sometimes in the ocean.

 This is 100% reproducible on realms.

Lava is the important part, I tried to put water instead of glass blocks and I would log back on the same block I logged out.

I already talked about this bug in MCPE-38374 but you asked for a new bug entry.

Since it might be relevant, I only tested this with a phone over cellular data.

More information :

https://bugs.mojang.com/browse/MCPE-47294?focusedCommentId=556137&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-556137