I had the same problem when I first brought my boy home. He had a small streak around his tail just bald. Took him to the vet, she told me that it might be fleas or something (i don't remember) and that she could give me a sample of trifexis once he's of age to see if that may be the issue.
Well, since then, I made sure he didn't bite on his tail and fed him the flea medication that the vet gave me and his tail is completely normal now. I actually forgot that he had the same problem til this post.