The experiences how the geniuses and other Apple store employees handle this issue is very different, I made these experiences myself. The only option for sure is to buy the phone at the Apple store. If it has the issue (or another one) just return it within the 14 days and buy a new one (maybe you cant do this anymore now, but it's my recommandation for any future buy). Unfortunately this issue is very widespread and even some iPad Air and Mini Retina are affected.