I can say first-hand that I've been sent back when I decided to play dumb and headed to the shortest line, which may have been for refunds I forget.
Are you certain that as a foreigner with a just confirmation code "in some cities around Shanghai, you do not even need a paper ticket anymore." At a minumum I believe you have to go through the photo session on the 12306 app. Not likely the OP is going to do this.
A friend of mine did it recently in Shanghai and Suzhou. Staff seemed confused about the QR code and the machines could not read the QR code on his phone. But scanning the passports worked fine (to enter and to exit).
They did not use 12306 and did not do the confirmation. They bought they tickets online with an agent.
Seems to still be work in progress as few foreigners use this method, the staff at train station is not sure how to handle this.