sbiddle:
What phones are you using with issues?
Scanning phone screens is a nightmare - you only have to look at the issues at airports. In recent versions of Android an app can force full screen brightness which is something the Air NZ app has recently started doing which is great.
Z aren't doing this with their app, but probably should be making the barcode slightly bigger and forcing full screen brightness when the barcode screen is visible.
Thanks for your help. My phone (on which the app works - with QR code scanned inside) is an old Vodafone Smart Ultra 6 and my wife's phone (on which the app is totally useless) is a 2018 V/f Smart V8. Both budget phones for sure, and aging too, but if my 5 yo phone works but my wife's newer one doesn't (not even inside), then it does seem strange???