AC flight's points credited to my United account even though Aeroplan is on my boarding pass!

I bought a YYZ-LAX round trip ticked from United, with the first flight being operated by United, and the return trip operated by AC.

First flight is no problem. Second flight on the ticket purchased by united is LAX-YYZ on AC metal. I called ahead to change my loyalty program for that flight to Aeroplan. Boarding pass printed at airport (which I'm holding in my hands) says 25K - P - AC*S. My boarding pass certainly has my Aeroplan # attached - I'm 25K.

I check my United app after over a week of not seeing Aeroplan points, and it got credited to United!

What happened?