UA is notoriously bad about posting correct bag info. when partner airlines are involved. However, AC is no slouch in bad policy/comms itself.

As you mentioned this was an award, meaning the flight you checked in for would have been AC-operated and marketed, meaning their bag fees would have been applied, assuming you flew them first. As an additional point here, IME, AC Transborder flights booked with UA miles roughly 2 years ago used to get a free bag, but no longer do. They have updated some things.

in order to really know what happened here, need additional information. What was your full itinerary and who operated which flights? Were any of your bags oversize or overweight, which would add additional amounts to fees charged. Without the info, no one can tell you whether the charge was correct or not.

You are supposed to be able to rely on the information UA provided you. If they displayed erroneous fees, they should absolutely make you whole, via a DOT complaint if they won’t do it on their own. If AC charged you the incorrect amounts, you should start with them, moving to a DOT complaint and/or CC dispute if they will not make you whole.