I do not know why the hair on his muzzle would do that. However, he is just about a year old, and their coats take up to 3 (or more) years to mature. Your vet says not to worry...but you do...so would I.
However...
I got Brody at 14 months old, and he had a fairly long top knot. His color was changing, then one day I noticed golden tan starting to come in at the roots of his hair...such a lovely 'glow' to his face.

A few months later, as the color was changing to the lovely golden tan, I came home from work and his top knot was gone...

The ends were a different texture...fuzzier...and darker golden tan. I had not done anything different, but, of course, I figured I had done
something wrong to cause it. Oddly, the silver colored strands that had not changed to tan yet did not 'fray' and break off, and the hair that broke off, well, I have no idea where it went...there was no 'clump' of hair anywhere that we could find.
Several months later, a breeder friend sent a photo because one of her girls had
her top knot vanish...

...but she still retained her long dark silky coat elsewhere on her body. Her top knot looked just like Brody's had, so I told her of my experience. In both cases, it seems to have just been a coat change...just seemed so sudden...abrupt...to both of us.
Brody now has a lovely top knot. The color is a beautiful light golden tan in between the first 'transition' color (platinum blond) from the black and the darker golden tan that 'frayed' and 'dropped' off.
I have no idea what the answer is, but I suppose it could be something along the lines of what happened to Brody and my friend's dogs' top knots, and it may re-grow, given time, too.
I am including 4 pix:
Brody, just before we got him...they had him puppy cut after this was taken...it is a 'bad' print I scanned back into the computer on 5/11/11...lol I would have asked them to leave him in full coat if I had known they were going to puppy cut him
the day I picked him up...especially since they
never had his ears trimmed, which I would have have the groomer do on every visit...and they took him monthly.
Brody, on 5/22/11...we got him on 5/14/11.
Brody on 10/27/11...note the 'platinum blond' top knot with silver strands and a lot of silver coming out of the top of the band.
Brody (blue hoodie) with Mia (red hoodie hidden behind her all golden hair) after a 'dirt and weed expedition' on 11/25/12...note the difference in facial hair color between the 10/27/11 and 11/25/12 pix, and that his top knot has grown back out quite a bit.
I, unfortunately, did not think to take a photo of the breakage, but he did not need a band for several months except for the few remaining straggler silver hairs, which eventually broke off as well.