I 've seen problems with romex being stipped with a razor knife causing a bleed through between conductors.It appeared that the wireman made two quick slices across the romex.When you fold it back and look you could see the cut's in the inner wire insulation.That and bare copper being twisted up tight upward toward the knicked insulation will cause a ARC fault.As stated earlier ---start from the line side and work from there.You'll find it.Make sure you have each AFCI neutrals isolated from one another and that each is working properly.