Just a few days ago I was on an AA mileage run LAX-SEA-GEG. Alaska cancelled the LAX-SEA segment, and rebooked me on the next flight to Seattle. Unfortunately, it continued to Spokane so I was actually booked as LAX-GEG and thus losing a segment. I called AAdvantage Customer Service, they checked that the flight was cancelled and immediately credited me for the LAX-SEA portion.