I’ve never gotten a boarding pass online for any trip that includes an international segment. For example, using AS miles I booked ANC-SEA-KEF. I could not check in online, and to obtain my AS boarding pass for the first leg, I had to go to the full service checkin area at ANC. Moreover, they had to manually add my KTN so I could use precheck, as it somehow didn’t automatically work. They also gave me a boarding pass for SEA-KEF, but as we all know it is useless as passport control at the boarding gate will issue a new boarding pass (which I still have as a memento).