IPv6 instead of IPv4, being sent to the server with priority.

Since the Nethernet update, the Minecraft client prioritizes sending the player's IPv6 address to the server, rather than their IPv4 address.

For a public server, receiving only an IPv6 address is terrible for moderation, as these change much more frequently than IPv4 addresses.

Another strange scenario is: In a home network scenario, if a server is hosted on a public IP (e.g., 201.XX.XX.XX, with a local address of 192.168.0.10) and you attempt to connect from another device on the same network (e.g., 192.168.0.20) by adding the public IP (201.XX.XX.XX) to your server list, the server will register the connection as coming from the local IP (192.168.0.20) instead of the public IP (201.XX.XX.XX).