What happened is that many phones failed to resolve sufficient detail at a sensible distance, so people have to get their phones closer to the QR code.
The combination of trying, sometimes several times, to grab the QR code, when it is poorly located, and holding other people up, even having to push back against a queue because it took a while to realise the grab failed, all end up with less effective social distancing.
Further, last time I visited a place which had a QR code, the venue itself couldn't care less if you scanned it or not. They just wanted your contact details (name and phone number) - which we were told was mandatory.
All a bit pointless if many visitors don't use the app.