Default proximity sensor behavior?

Okay, so I upgraded to a new iPhone 5 on Dec. 7th. Seemed like things were working fine until I had an extended phone call with an automated answering system, requiring number pad input, etc. When I was making the call during the first set of input questions, the phone worked as expected. It dimmed and the blackened the screen when I placed it up to my ear. It then brightened again when I removed it to input numbers for the automated call menu. Fine, right? Then after about 30 seconds more, I needed the number pad again, only this time it did not brighten again when I needed it--the screen remained black after removing it from my ear. I had to push the home button to make the menu reappear. Kind of annoying, especially when trying to end calls. Is this a new default behavior? At first, I thought not and an Apple advisor thought so, too, sending me a replacement. Unfortunately, I refused the replacement because it arrived looking like the back of it skidded across concrete and being told I would only receive a refurbished product (this was only 4 days after purchasing the phone, but have AppleCare + anyway). Anyway, long story short, I have the same new retail phone I bought. I called again and the next Apple associate said the behavior was completely normal and that I had no need for replacement anyway (i questioned the refurb thing, too, but was told that was how it is, even though I got my old iPhone 4 replaced with an in-box retail phone when it had a speaker issue). So, what exactly is the default behavior the proximity sensor is supposed to exhibit? I hope it is acting normally, as I really don't want to have to speak with Apple again and be told I will only get a refurbished product. That will definitely dissuade me from services and devices in the future. I would think if retailers are able to replace an item with a new one if within the return policy (not to mention 4 days), then Apple certainly should.