Teambuilder with multiple people on same network.

Firstly, this has been happening for many patches.

When I create a teambuilder lobby with a friend sitting next to me, after about 1 minute, I (the team leader) am kicked from the lobby, however other players are just sitting there as if I haven't left. This is purely in teambuilder, we can enter all other (non-ranked) queues as a party just fine. We've both temporarily disabled our firewalls and AV for the duration of the lobby, but it still closes after about 1 minute.

I've also checked the router settings, and there are no restrictions that would affect LoL traffic.

Cheers UV