It sounds to me that during the booking of a Business reward ticket if there is an AC segment and a UA segment, the AC segment will be in AC Business, but if the UA segment is a 2-class plane, you either get an economy seat, and manintain your Business redemption cost, or if you want F (which is really just Business), the ticket is now a full First redemption.

But that's not what I experienced with my booking.
In my situation all segments are classified as Business, three on AC, and one on UA. This implies that the UA flight has 3 classes because it let me book a Business seat. However Seat Guru tells me it will be a 2 class plane.