I've started playing again a month ago and just now started seeing this. There have been multiple times where I or one of my friends die while playing together and there items and XP just disappear. Here is what specifically happened to us. While in a cave directly below our base my friend died to a spider, no creepers around. It took them less then 30 seconds to get back down there but there was nothing there. Another instance was I was poisoned by a witch so I blocked myself into a little hole so nothing could get my while I healed but somehow got killed by a zombie while in my hole. Came back 1 minute and 27 seconds to the dot. I kept track because I was afraid it would happen again and it did. Not a piece of my stuff or of my 50 levels anywhere.