OK, got my 2018 MBA about 2 weeks ago. Wanted to do a Signature capture with Preview, obviously needing the camera to do the capture. It will not, while my 2011 11" i7 MBA will under the same lighting conditions. What's apparent is that the 2018's camera is not adjusting exposure like the 2011 is. Some of you who still have yours might want to test this.
I was googling to see if anybody else had this issue, and I stumbled onto this. I'm going to visit the Apple store tomorrow to see if I can get this replaced, not looking to get a better video through the camera, but to at least get the signature capture feature working.