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. It makes sense that it might. Unfortunately, this may be a condition that is more related to thermal changes, that to sunlight, so there is also a good chance that wide spectrum lighting wouldn't help. There are no scientific studies that I am aware of that support this notion but it seems reasonably safe to try Vitamin E. Phemphigus foliaceus is an immune mediated disorder that German shepherds are somewhat prone to.