When a player is joining a world their inventory isn't saved and everything is lost, only the one hosting the world has their inventory saved
Linked issues
is duplicated by 21
MCPE-2177
When Playing Multiplayer Guest Looses Inventory Upon Leaving
Resolved
MCPE-991
When a player does not own a server, disconnecting from the server and coming back causes the player to lose his entire inventory and spawn back at the server's original spawn.
Resolved
MCPE-544
Multiplayer leaving and joining bug
Resolved
MCPE-734
When player 2 leaves the game he/she will use their items
Resolved
MCPE-2064
Items lost when somebody log out
Resolved
Comments 6
This is when reconnecting, and we are aware of this issue. The thing is that the server currently doesn't identify players. This will be fixed at some point in the future.
Richard, what you are describing is entirely unrelated to this issue.
I am a well-established player on my community server. I logged off on Friday evening, and logged back in shortly afterwards and I found myself at spawn with none of my items on me and my Enderchest empty. I did not join as a new player. Rather, my character data was affected by this bug which screwed me over.
I'd prefer that Mojang raised the priority of this bug. It can be quite crippling and frustrating, especially when it happens twice in the span of two hours as it did to me.
This is not a bug... If you join another players seed of course you will not have anything in your inventory. Get your friends to craft some extra tools then join them.