Big time bug. Wife logged out for a couple hours, and when she logged in, her inventory was reset to what it was early this morning when she first logged on. Lost 16 full stacks of Diorite. Just did a test myself, put a bunch of stuff in a chest except for one new thing, logged out, logged back in after a few minutes, and my inventory was set to what it was early this morning. Also, when we looked in our inventory after logging on, we got the inventory achievement.
And on top of that. When we logged out and logged back in, we were at the spot in the world where we had logged out at last night.
note that changes in the world that were made stayed the same, ie dug out parts stayed dug out, built areas stayed built.