I have been trying to figure what could cause this but I haven't figured it out. There is no way for me to fix this unless you describe exactly what you did before you left, when you left, what happened when you came back, what map you played on, what time, etc.
Similar thing for me.I left to rejoin for gold bars, but when I came back my mines were gone.The only thing I did differently than usual, was that I used the inventory filters in a line on the top of the inventory. I'd never used them before, so I was checking them out, and I used the mine one a few times to look at just my mines. Not sure if that helps or has any relevance, but like I said, it's the only thing I did out of the ordinary for me.
Player joins the game.Server fetches information from database server DB.Server stores information on each player object (serverside).Server tells Player information (server -> client).Player does something: Client sends information to Server.Server updates information in DB.