Farriers formula is magic ! Makes their coat shiny too
I had this problem an used Durasol it was fantastic.
This happened to my mare when she was a weanling. Farrier suggestion was use a shoe nail or a really fine hoof pick and keep the area between hoof wall and sole really clean and douse in betadine twice a day. Eventually hers grew out.
Keritex (i think that's how it's spelt) hoof hardener