Y axis glitch

I started a new world I did all the normal things the only thing that was strange was the seed that had a mesa next to a desert right on spawn, then after getting some materials I went mining.

 

The caves were perfectly generated and there were no problems with that, I logged off still on the cave.

 

When I joined back about 2/3 hours later, I was on the surface I tried to remember if I left but it was clear that I logged off while I was still mining.

 

There were no tunnels that I could have used to get to the surface nothing was there and it was on the same X and Z coordinates than when I logged off but the Y changed.