I received the latest iPhone update today and later in the day, when trying to check-in for tomorrow's flight after the 24-hour mark, nothing I did (page refresh, going to accounts to sync my trips, etc.) would make the check-in button active on the flight screen. I finally ended up going to the "More" screen, chose the check-in option, entered the confirmation code, and I was able to check in with no problem.

Even after doing that, the check-in button on the actual flight screen still isn't active, even though the boarding pass button now is (since it issued a boarding pass through the backdoor approach). Weird.

A little gremlin in the system after this last update?