Just going to correct Bob here.
Bronze player COULD get matched with a platinum player in some intended edge cases. However, Bronze player should NEVER be matched with a Diamond+ player.
We investigated this, and in all the cases we checked, it is a Gold Player in a premade with a Bronze player that gets placed into a Gold + Plat lobby.
This is intended BECAUSE:
Gold players can duo queue with bronze and silver players. When you are in a premade, you play at the highest player's tier. In this case; The Gold Lobby.
Sometimes when queue time is sufficiently high, lobbies starts merging to kick off games. This is a compromise in the design because we don't want these players to wait forever. So sometimes during off-peak hours, this starts happening. This is fully intended.
With that said, this feels like a bug, and I'll look into adjusting this specific edge case in the future.