Interesting. That wasn't my experience today. I just walked up to the United Premier Access line to check in and check my one bag. She asked to see my COVID test result paperwork, which I handed to her and then she handed it back. She then gave me my boarding pass and a paper form/health questionnaire to fill out. I filled it out and got in the security line and handed it off to the person checking boarding passes before walking through the door to put my carry on luggage on the X-Ray conveyor.
Maybe the electronic systems were down. It would make sense for them to have a paper process in place for that.