NetherNet allows only one player per port on Bedrock Dedicated Server

Short description of the issue

When using transport=nethernet, Bedrock Dedicated Server appears to allow only one player to connect through a single UDP port mapping. This occurs even when max-players is configured to allow multiple players. Allocating additional UDP ports appears to allow additional players to connect, but this is a workaround rather than a proper fix.

Steps to reproduce

1. Set up a Minecraft Bedrock Dedicated Server using a version affected by this issue.

2. Set transport=nethernet in server.properties.

3. Configure max-players=20 (or another value greater than 1).

4. Configure a single UDP port mapping for the server using server-udp-ports, ensuring that the port is correctly allocated and accessible.

5. Start the server and attempt to connect with multiple distinct clients.

6. Observe whether a second client can join while the first client is connected.

7. Allocate additional UDP port mappings and repeat the test to compare the results.

Expected result

Multiple players should be able to connect through the configured server networking setup, up to the max-players limit, without requiring a separate UDP port mapping for each player.

Actual result

Only one player appears to be able to connect through a single UDP port mapping. Additional players cannot join as expected, despite the server's max-players setting allowing multiple players. Allocating multiple UDP port mappings appears to work around the issue by allowing additional players to connect.

This behaviour appears to be related to the NetherNet transport implementation or UDP port/session allocation. Further investigation is needed to identify the underlying cause.