I have a J award for SFO-HKG-BKK (CX873-CX755) booked and am trying to add a positioning flight from MSP-PDX-SFO (AS426-AS1044) to the reservation. When I called in the agent seemed to think it should work, but ultimately couldn't get it ticketed after multiple attempts - something about the connection not being a valid pairing. They said they could only add the positioning flight if I did an overnight layover in PDX and left for SFO the next morning (AS426-AS1212), but it wasn't exactly clear why. Any idea what is going on here?