I mean, there are no hard set “rules” one must follow as a fan and I assume the loser of the IU/ND game will likely pull for the winner to go on and win it all.
I loved the Butler basketball story when they were the little guy but never considered myself a “fan.”
Pulling for someone to win is normal and as I turn on games in which I have no rooting interest as if doesn’t affect my favorite team, I often find myself siding with a team based on various things as the game progresses.