Granted, my experience is with ORD and EWR, but I've never had issues with the UA desk post-Customs issuing new bag tags for my luggage. In my experience, SK has been notoriously bad at checking through bags if I had a separate, same-day UA domestic ticket; they simply refused, printed a tag to ORD/EWR and said, "You need to pick up your bags there anyways." Once I arrived at ORD/EWR, we would take our bags to the UA desk, show our boarding passes, and they tagged and took our bags. Once we were even able to take an earlier flight.

I'm honestly surprised someone in Terminal C accepted the bag since it was over 8 hours until the next flight.