I had the monster chosen as my third option (Assault > Medic > Monster > Support > Trapper). Two games in a row, I was the monster. These are two separate games, as in I left the lobby after the first round finished, and found another.
After being chosen as the monster, again, I rearranged my roles so the monster was last (A > Me > S > T > Mo). Four more games followed, four more times I was the monster.
The matchmaking, and role choice, doesn’t really seem like it does anything.