If a player on the same device as the server joins the game and leaves, that player won't be able to join until their Minecraft UWP app is restarted or the server restarts. This can be observed in the video attached.
The console does log that the player has left the server and no additional messages are logged when attempting to rejoin.
Update
Disconnecting from any game whether locally hosted, network hosted or on a server, the player is unable to reconnect to a server hosted on their device.
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Comments 6

Do you connect to the server using the Friends / LAN Games Tab or do you use the servers tab?
I think I usually use the servers tab and connect using 127.0.0.1 / port 19132 to hit the local server without problems.
I too have noticed that if I use the LAN mode it does not work 100% of the time if I'm running the dedicated server on the same host.
This is a very simple workaround if it works for you.
I'm not able to reproduce this.
I grabbed the latest release and ran with default settings. I was able to connect, disconnect and reconnect 6 times before I gave up.
Have you run the command in the help file referring to UWP app isolation?
I'm not able to reproduce this.
I grabbed the latest release and ran with default settings. I was able to connect, disconnect and reconnect 6 times before I gave up.
Have you run the command in the help file referring to UWP app isolation?
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki