Players outside my network can join my lan game!?

I tried to setup a blitz lan game with 3 players.

Here's what happens:
- I create a multiplayer server, Blitz, LAN checked.
- The players on my network join.
- The game starts
- A random fourth player outside my network joins ...?
- After a few minutes the fourth player gets kicked along with a player on my network ...?
- After about 5 minutes random players outside my network join again.

The game works fine after that... except that there's players outside my network in my lan server ...?

Facts:
- This has twice, the only 2 times I tried to host a lan game.
- My ports for online games are open.
- The player in my network that gets kicked is on the same network switch as me.
- The player that doesn't get kicked is on the same router as my network switch.
- The kicking also happens when I host a normal multiplayer game.
- The computers on the switch have static IPs.

Am I doing something wrong or is it a bug?

EDIT:
added DxDiag.

EDIT2:
With kicking I actually meant: Losing connection.

Very strange indeed. We would love to see the launch.log files from both the client and the server for one of these occurrences.

Edit: Just reproduced this. Looks like a genuine bug. The LAN flag isn't respected for Blitz games. We should be able to have that fixed by the next patch.