I was in the mine (about Y:10-20) and i quitted the game. When I came back and loaded the world, I spawned in exactly the same X and Z, but I was on the mountain.
Game probably didn't save my Y position correctly. I can't redo this bug, but I'm sure it will happen again sometime.
I was trying to do 404 challenge (my way 🙂 ) and I quit the game, like 1 hour later I start playing and I spawn in the sea with X and Y coordinates the same. Same as you but im in 0.7.1 (iOS)