I had a similar crack on my iPhone 6, and during a genius bar appointment I had an amazing rep who realized it was a "pressure fracture" and replaced my Phone with a brand new one
The caveat was I had Apple Care Plus, however this was not counted towards one of my phone replacements (I still have both replacements left)
If there are no signs of dropping the phone, it can be a fault of the phone's. Hopefully you have apple care and/or an excellent customer rep!
Some rep's don't try to do a damn thing about this stuff, but other reps go above and beyond so hopefully you get one of those OP