chests disappear

after about a week of play chests disappear and contents removed

What I expected to happen was...:
the chest to stay as it is

What actually happened was...:
when entering game chests had disappeared - clicking on the space where they should be the chest came back into view but all contents removed - very annoying when been mining all week!!!

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. close down game
3. start game
4. chest gone