To my knowledge, Historical is actually a subcategory. Like you have the main category ROMANCE, then you would write HISTORICAL ROMANCE. Or you might have ACTION ADVENTURE, CIVIL WAR HISTORY, HISTORICAL.

If you think about the old days, unless you were writing a HISTORY BOOK (which would be Non Fiction), historical was shelved with the mother genre. You have “The Great Gatsby” and “The Count of Monte Cristo” shelved in general fiction, even though nowadays (when using an algorithm) we would add those additional descriptive tags.

But Historical is a descriptor, not a genre. Does that help? And if you aren’t selling books, you might need to look at how your book is listed and if the algorithms are picking it up. That can be a meta data problem.