This is there because the player could have requested for 2 parties, joined the third one, and in the queue for 3 mins, then party 1 invited them. This would mean they will automatically get pulled out of their current party and instead join party 1. It definitely could be improved, but it's not very straightforward ...
well said Rivi. I agree that the fix isn't super easy, but it's definitely a QOL thing we can look at!