Lost inventory when logging onto world first time but not second.

I recently logged onto a few of my friend's worlds where I was back at spawn and nothing else. I thought this was a fluke as they had only recently updated and nothing major was lost. However, I just now logged onto my own survival world to find I was back at spawn with all inventory lost except a new empty map. All buildings and mobs were still intact, but I had lost very valuable items I had spent hours to get. I panicked and so quit the world before joining again, by which point I was back at my base with all items returned. This is obviously not a fluke by now as it has happened a few times (on a friend's world and mine). If you have any advice I would be very grateful