The last time I did that the TSA scanner was able to read the QR code just fine, but the tablet app that the flight attendant was using at the gate couldn't read it for some reason. After about 10 seconds of fidgeting with the tablet they asked me what my name and seat number was. I told them, and after checking the list they let me through onto the plane. It looked like they tapped around in the app to override the QR code scan or something.
Fast-forward 20 minutes, and we don't push back from the gate when it's time to depart. After another 5 minutes of delay they got on the PA system and said something about the passenger count being off and that the airline's headquarters wouldn't authorize departure until they figured that out. At one point about half an hour into this a flight attendant walked over to my seat and leaned over to adjust the air flow thingy, which I thought was a super weird and random thing to do. In all it took nearly an hour of everyone sitting on the plane at the gate before they figured it out and authorized departure.
I actually have no idea where the breakdown was, because this happened at the gate when I flew earlier and it wasn't at all a problem. I presume the flight attendant scanning QR codes at the gate didn't hit the right buttons on their tablet that time. If we're going to rely on peoples' completely random personal devices to track authorization to travel, our systems need to be a lot better than this. Exceptions to whatever they think should be the "typical" flow should be straightforward and streamlined.
In the meantime since they've gotten rid of kiosks in my local airport I guess I'll be going to the front desk every time and ask for printed boarding passes.