Granular loss on 6 year old roof

Not sure if this is a manufacturer defect. Observed the edges of many of the shingles with granular loss. A couple of shingles had significant granular loss. This is a 6 year old roof.

How was the attic ventilation?

static, power vent and soffit.

No concerns when inspecting attic.

Are those Owens Corning shingles? Looks so from here.

The surface layer of asphalt is not strongly, uniformly bonded to the mat, so where people walk, it separates easily at shingle edges where the force of a footstep is concentrated in a small area when that part of the shingle is stepped on. When the middle of the shingle is stepped on the pressure is spread over a wider area and the surface layer of asphalt is less likely to break loose from the mat.
Except where you get a shingle where for some reason the surface layer of asphalt is very poorly bonded, and then a big flake may come loose.

Where does low-quality by design end and manufacturing defect begin? The former is intentional and latter is not, but when intentional low-quality design leads to premature failure, it doesn’t really matter; premature failure is functional damage and there ought to be recourse through the manufacturer if the proper installation and notification procedures have been followed. You never really know what those are until you read the manufacturer’s requirements, unless there are jurisdictional (like state) consumer protections laws that take precedence.

How was the adhesive strip bonding?