My friend and I cannot be in the same game/lobby together which we can't seem to figure out why.

There are three of us at the moment and any combination of two works - aside from myself and Friend One. If Myself and Friend Two play a game then we can start the missions and kill thing without issues; If Friend One and Friend Two start a game then they can play together without any issues. It doesn't matter who hosts the lobby because we can't be in the same game without me failing to load into the mission.

It can't be networking because I can play with Friend Two without issues. And, it cannot be them having networking issues because they can play together without issues. We initially were having problems since two of us had made seasonal characters and the other had not but we fixed that and all went non-seasonal.

Any ideas why this would be the case or troubleshooting ideas we can try?
I thought it might have been due to me running the game in linux via Proton but that doesn't explain why it's only him.