You're correct. It is a bug in the original (and thus any subsequent mods). A couple times a year, my friend and I would play as teammates, and sometimes totally forget that when we won the 1st playoff game, it would switch to head-to-head. So we would just start over, and cycle thru the team that we thought would advance, and just keep playing that way for each round.
It's too bad, because it is a cool mode instead of always competing against each other.