I always always screenshot my QR code and present that at the gate. If we had rolling codes locked away in a proprietary app, I guarantee boarding will grind to a halt as there always seems to be some sort of access issue whether it be from the servers, user error, connection issues, etc not to mention people often have their hands full of crap which makes clicking through a bunch of app menus difficult.
The LAPD did one recently on a medical imaging office because apparently it’s suspicious that an office full of XRAY and MRI machines uses a lot of power.