My computer and my friend's mobile phone were connected to a same LAN. At first, in the "friends", he could see which world that I was playing, as well joining in my world. It seemed that everything was right.

But when the screen of his mobile phone was closed by him by accident, he was forced to leave my world (because the Minecraft on his phone was stopped). When he reopened Minecraft and tried to join in my world again, it was found that he couldn't see which world that I was playing! That also meant that he couldn't join in my world.

We checked all the settings but found no wrong settings, and we also reconnected the LAN. But he still couldn't see which world that I was playing.