IPhone 5 screen goes black after extended call?

My iPhone 5 screen goes black and locks after I finish an extended phone call, which is about 10-15 minutes, instead of launching back to recents. It doesn't do that with short phone calls. In general I just press the home button to turn it back on. It doesn't happen often, but has happened enough times to where its become an issue to me. I asked a an Apple rep at Best Buy and he informed me iPhones do this to conserve battery. But I find it unlikely, because my other iPhone didn't do that. Then I was told by someone else that if you're in a dark room, that throws the sensors off. I really don't know about that either. 2 out of the 4 times it happened I was in a dark room, what about the other 2?

Does anyone know anything about this particular subject?

Try going into settings > general > and see what auto-lock is set at. Try setting it to never and see if that solves the problem.
Also make sure your phone has the latest version of software which is 6.0.1
You could also try to exit out of all your running apps by pressing the home button twice. Those apps can sometimes cause conflicting problems.
If that doesn't work you will probably just need to reset all settings under settings > general > reset

If all those fail the only other trouble shooting option is to create a backup of your phone then do a factory restore from backup, just to reinstall the firmware

This doesn't sound like a hardware problem so one of those options should fix it.