I am based in Canada and United has my Permanent Resident Card Info recorded! (as well as my Global entry via Nexus).

Not sure if that's a requirement to get on a plane heading to the US for Green Card holders, but when Canada reinforced that Airlines should check the status of permanent residents entering Canada (no card, no flight without exception), I suffered a lot on both Air Canada and United for quite some time!
United was taking me all over the world without any issue thanks to Visa Waiver, but when it was time to enter Canada, it was a nightmare even at airports I couldn't use check-in machines (same thing with both United and Air Canada, I had to physically show my PR card to an agent who were struggling as well to complete the check-in; it was very frustrating).

Eventually, United implemented a placeholder for the PR card that is recorded since. And whenever I am flying United to Canada it's seamless (I am not eligible for online check-in on international flights, but inside the airport machine, I just click next next to get my boarding pass).