Cant Connect to other players world using your original map

Problem:

Player A created a world and sends a backup save file using backup world in settings to Player B.

Player B opens file  (.mcworld) sent by Player A and starts playing the world.

When Player A tries to connect/join to Player B in the same world a message will popup saying "unable to connect to world".

 

But if Player B change into a different map Player A can join. 

vice versa