My sprocker has something similar on both front paws between the back pad and the outer toe, and it has been ongoing for the last two years. It looks like a rub or contact blister (similar to what you would get on your heal in badly fitting boots) with the top layer of skin. being rubbed or sloughing off over an area of about 8mmx8mm, sometimes on both surfaces. The vet has checked him over several times but really no wiser. Sometimes exercise seems to rub it raw and peel the top layer of skin off, sometimes he can be walking for several days and no issues. Sometimes he licks it raw himself. No grass seeds or splinters. Same position on both paws. Pads aren't hot.
We have tried changing his food, cutting out any dairy products, had his anal glands checked (impacted AGs can make them lick their paws, apparently!), his coat is clipped short to keep him as cool as possible, we put socks on his paws overnight and when unsupervised to try and stop him licking them, we wash his paws after walks, he has had his claws clipped, anointed the affected areas with various veterinary creams .... all to no lasting effect.
Both us and the vets are running out of ideas!