The QR is regenerated every time you open the app, and probably expires after a small amount of time. Amazon probably does this to 1) avoid people sharing code

The QR is regenerated every time you open the app, and probably expires after a small amount of time. Amazon probably does this to 1) avoid people sharing codes with each other (especially non-Prime customers) and 2) the code can be linked to your payment so someone could possibly steal your code over your shoulder and start buying things. I don't believe Amazon needs the app to track you in-store. They have enough cameras and wifi APs in the building to do that.