There are no consistently successful treatments for this condition in the literature but this condition is not harmful to the dog. This is depigmentation of patches of skin, leading to patches of white hair in haired areas and to pale skin in other areas. This can affect the nasal planum. It is not common in German shepherd dogs, as far as I know. There are no consistently successful treatments reported for this condition, either.
Contact allergies can cause depigmentation of the nasal planum.
The most common contact allergy to have this effect is an allergy to plastics. It is worthwhile to try stainless steel or ceramic food and water bowls to see if changing types of bowls will help, if a dog with this problem usually eats or drinks out of a plastic bowl. Some dogs lose pigment in the winter time and have pigment in the summer months. There are no consistently successful treatments for this condition, either. There are reports of occasional success using the types of treatments you have already tried, but truthfully I suspect that these are just spontaneous remissions of the condition.