Player 2 has issues after a party of 4 is made

[(NSW)Zieghash]

So my partner and i have been having an odd issue for quite a few months now, honestly started happening since the first game compression update.

When we are in a party of 2 or 3 the game runs as expected but the moment a fourth player joins the one who isn't host starts to experience problems, specifically player 2.

The main issue being really bad lag to the point that the game will be at lest 20 seconds behind ( we counted ), strangest being that the other players in the party are moving and performing actions as usual but the enemies and even countdown timers are delayed meaning that a defence timer for example will countdown and have the decision made ( to leave or stay ) before player 2 even sees it meaning they get left behind on extraction or just flat out kicked once the rest of us leave, this include a hosts migration.

Another is that once a fourth person joins player 2 will freeze or hitch a little or a lot then be kicked, trying to rejoin the party usually results in player 2 being told the party is full or rejoining the party only to be kicked immediately again.

So usually i am host and my partner is player 2 but we have swapped it around for testing purposes and have found that the host is fine and that i get the problems instead.

We are on the same network, we have tried port forwarding, we have tried re downloading the game, we've even tried changing what region were connected to. No NAT issues visible either.

We have noticed that after every update the issues have been getting worse going from an occasional annoyance to almost every mission and at this point have no idea what could be tried to remedy it. Any advice or help would be greatly appreciated because as it is we cant play the game together anymore without restricting how many players we can have.