All's well that ends well, and my bag made it onto the belt at SIN, although not in the first round of Priority tagged luggage, where it should have been.

I learned a lot from this thread - that Amtrak is still running Arrow, and that YVR's computer interface appears to have problems with United's check-in stands. I still wonder why this problem was confined to United and all the other carriers were checking normally, as far as I could tell?