The "UWP Default Outbound Block Rule" of the Windows Defender firewall blocks all outbound traffic from the app. The Event Viewer reports a constant stream of events 5152 (Filter Platform Packet Drop) and 5157 (Filtering Platform Connection) failing the rule.

Adding a manual rule for the AppX package doesn't work, likely because the UWP default rule is more generic and takes precedence. This prevents any Win11 machine from joining a LAN game.

On Windows 10 however traffic goes through and the LAN game appears. Verified on at least 2 separate Win11 machines, one of which was connecting to the LAN game before upgrading from Win10 to Win11.

It's unclear if there's a UWP capability missing nor what's the difference between Win10 and Win11 here, but this effectively blocks all LAN games.