Floating island disappeared!

I had built a floating island in one corner of the world

What I would expect 

I would expect the built environment to stay the same from game session to game session.

What happened

The floating island disappeared completely

 

What I was doing 

I was playing game in multiplayer on LAN with sons.

Did a 'save & quit' and next time I entered game the island wasn't there.

All other structures remained the same.