It depends where you bought the racket, how long you have owned it and if the string tension exceeds the maximum tension of the racket.
It looks like high noon disease, but this happens normaly exactly in the middle. Maybe racket stretched too much with no loadspreader or too high tension on the crosses or the racket got any damage due not properly cut strings after breakage and rest some time uncut in the bag. Could also be a sunken grommets. The grommets look a bit worn down. What was the tension? I personally think that it isn't a clear warranty issue, because there a few reasons which could explain it.
The racket was brandnew? 26 lbs don't cut so deep into the grommets. If brand new hit the shop and show it. If not, hit the stringer and show it. The Yonex grommets wouldn't looks so notched after a 26lbs job. I suspect a fake racket or sloppy stringer according to your infos.
I ORDERED ONE FROM Hong Kong AND I STRING IT TO 28-29 LBS WITH BG 80 POWER. THE IN BETWEEN THE TOP 3 PAIR OF GROMETS THE FRAME CAVED IN IT WAS SP DESIGNATED ONE. I CUT THE FRAME CRODD TO EXAMINE, AND FOUND OUT THAT THE THICKNESS OF THE TOP OF THE HEAD WAS MISSING GRAFITE LAYERS. IT WAS VISIBELY THINNER THAN THE REST OF THE FRAME. ALL LABELS WERE AUTHENTIC. DOES YONEX FAILING QUALITY INSPECTION???
