The problem is the mandible isn't extremely flat to begin with, and there is already a mentolabial fold. Advancing will deepen this, although lengthening should also help to curtail the issue. You can also get a little HA paste added to plump out the region and reduce it. I had a 7mm genio recently, my mandible was very flat to begin with and the fold was almost none existent, but my surgeon said it looked deep after advancing, so he added the paste.