Okay, here's what happens:
Let's assume everyone queues up as solo, and ignore MMR to make it easier to explain.
A player queues up with two roles. The system puts them into their primary role.
Second player enters queue. System gives them their primary, unless it's the same as player 1, then it gives them secondary.
Player 3 enters queue with primary/secondary that does not conflict with players 1 and 2. System gives them one of their roles.
Player 4 enters queue, same situation as above.
Nobody enters queue with support or fill for a short period.
The system wants to get this game started, and luckily player 5 just entered queue. They're grabbed and tossed into whatever position (likely support) is missing to fill this group out and get them into game.
A group waiting for a fifth player has a greater need to fill their team out than a player freshly entering queue to get their preferred roles, once a certain duration has passed.