It has recently happening frequently to us that when everyone signs out of the realm and then we come back online the work in the last couple of hours are reverted without the game having crashed. 

The screenshot above is one of the examples when the last save is 2+ hours before the last person signing out of the game.

UPDATE: It is unclear if the game not saving consistently when the last player signs out or if the saves are silently failing. Below is another instance that many hours of gameplay was not saved even though players were active on the server, but after everyone signed out and someone signed back in, the game was reverted and then an old snapshot is saved (the "28m ago" save is when someone signed back in).