Pieces of built things would move or go missing!

Sometimes when I create a new world, and build stuff in it and log out and back in later. stuff built would move somewhere else nearby and swap places with where its moving to. in some cases, areas on where something was built on would reset to what they where before something was built on it! In the first Photo you can see what I built was cut out in a perfect square, and in the second photo the Arena's floor was cut out and part of a house and a road was moved to the right of the arena, and the white floor you see in the third photo is suppose to be in the arena and the missing haft of the house is suppose to be with the other haft!