FWIW, when my son draws on his fingernails, (which isn't all that often) it usually means he is stressed and/or anxious and is using the sensory input to calm himself down. Telling him to stop typically just interrupts it for a few minutes, and then makes it worse, which can be interpreted as defiance by people who don't know what he is doing and why.

The school's "psychologist" (not licensed) in our district who did my son's evaluation when he was 13 told us she didn't see any signs of AS or any other problems, but that he was "rude and defiant" because she had to "keep yelling at him to stop drawing on his fingernails."