Strange networking issue possibly regarding Windows 10

[AdamN]

Ugh.. on a second thought, always assuming the source port to be valid won't fix this issue entirely...
because the way AdhocMatching worked is, when someone created a room they will broadcasts that room data to all members in the same group, and when that member received this room data they will show it on screen.
However, this broadcast is using a predefined port (initialized by the game when creating AdhocMatching) and all players supposed to use the same port, but due to this "incorrect port" issue some players might not be able to received this room data, because it will be getting a random port and other players won't know the port (not even the player that had this issue know which port they're actually using, because trying to retrieved the binded port will show the predefined port, which normally to be correct but may no longer correct due to this issue)

So, basically, always assuming the source port to be valid will only increase the possibility for AdhocMatching to work, but not entirely fix this issue, because the issue is on the router side, and we don't know what's triggering this behavior on LAN... may be enabling UPnP when playing on LAN causing some router to get confused.
But even if we can fix this on LAN (ie. by disabling UPnP options, that is if UPnP causes some router to get confused), people who use UPnP over the internet will also have a possibility to get the same issue due to port re-mapping done by the router or ISP (which is normal over public IP, but not supposed to happen on LAN IP)

PS: GameMode is also using a single predefined port to be used for sending & receiving data (as i remembered), thus will have similar issue to AdhocMatching.