Carol. Thanks for the suggestion. Yes, I've done that, but I have still lost all of my items and XP.
Chris, you said this so much better than I did. This is exactly what's happening to us as well. The game is unplayable (at least for anyone other than the primary player) because we keep losing our items and experience.
I have Minecraft on our Xbox and was playing as a second character in my wife's world (split screen). The update that just happened caused the issue that it was resolving for others. My character that had a lot of experience and inventory is now at the world spawn with no experience or items.
Just to add my experience. My loss of items was caused by the update. Both of us were signed into the Xbox before launching Minecraft, and we still ran into the same issue (even after closing and reloading 30-40 times). It sounds like both being signed is is working for some (which is awesome), but it did not work for me