I was just looking at my MileagePlan activity and noticed that on my roundtrip flight (SFO to SAN) back in February 2020, I was only credited mileage for the outgoing flight (SFO to SAN) but not the return flight home (SAN to SFO). Both flights were supposed to be an Alaska Airlines, but on the return flight, there was about a 2.5 - 3.5hour delay (my memory is hazy) on the Alaska flight. So I had the gate agent re-book me on a United Airlines flight (SFO to SAN) that arrived 2 hours later than the original Alaska flight.

I tried to fill out the "Mileage Credit Request" form on the Alaska website based on the Alaska ticket number, but it says that "This trip has already been credited. Please check your ticket number and submit again." (maybe because the SFO to SAN leg got credited??). There is a longer form to fill out, but there is no option to input United Airlines as a partner airline.

What is Alaska's policy typically on a situation like this? Did I lose claim to any mileage from Alaska by switching to the United Airlines flight? My United Airlines account did not get any miles for that SAN to SFO flight either.