Things disappear while I'm logged out

I placed a chest and put all my stuff in it. When I logged back into the game it was gone. This is a single player game that I am running on my home computer. Lost a lot of stuff.

I've put another chest where the last one was and it disappeared too.

The coords are 2002, 74, 2002 and it appears anything I place there disappears. I put a flower there after the first two chests disappeared and it vanished as well.

Further testing shows anything I place anywhere in the area disappears. I have deleted the map and have had to start over.