Read here: Died but your tombstone is not on server
This bug comes up alot and it is because your client and the server isn't talking to each other at the moment. Which means the server don't even know you died.
Usually when you die, you are busy with something and don't look at the "disconnected" icon on the screen showing.
If you are unlucky, the disconnect only happen just as you die. Because when it happens, you notice it. When you´re not busy with death and dying, you normally don't notice the short disconnects.
This issue isn't because the game programming make you lag or temporarily fall out of sync.
The client always tries to re-join when it disconnects or cannot get a response to the server. This timer is set to 30 seconds. if it doesn't get a response from the server within those 30 seconds, it disconnects and you end up back in the menu screen usually.
We are looking into solving this (as all bugs) of course!
The fix right now is to go into a single player world, and just respawn your items back.