Solved
Filter for Game In-Progress
We need a matchmaking filter or setting that blocks attempts to join Coop games that have already been started.
That would be against the general principle of the game's design to allow people to join in-progress matches - same design for numerous multi-player games including Battlefield 2042, V, etc.. It is highly unlikely this would be implemented, particularly for this game, which is now over 5 years old.
