I can confirm this is still happening as well, we encountered this yesterday while doing some performance testing.
Since the “Join Game” option is not shown, even for codes with social joining enabled, the only way to join the game is to load in after joining the party. However, since the max party size is 16, this makes it difficult to test with more than this many players.
We found a workaround by loading into the game with a full party, kicking people from the party from inside the game, and then inviting more players to the party. Since players stay in the game after being kicked from the party, the new party member can then also join the game, bringing the player count above 16.
In order to get an accurate gauge on server performance, it is critical we test with a full lobby. Since the game we are testing for will potentially be more than 16 max players, this confuses that process and makes it more difficult than it needs to be