Sometimes, in game, the server host is invisible to the clients. The host is in game, playing, and can place blocks, but isn't visible to the other players. The blocks he is placing can be seen by the clients, but he cannot. The host can be seen once again if the client disconnects then reconnects.
If you did not already, please remember to perform a search before creating a ticket to see if your bug hasn't already been reported.
This issue was reported previously as MCPE-550; comment your experience there!