I discovered that MA1A has both DB032240 (Ben Hur) and DB109202 (Miss Delightful: Mischief in Mayfair), have American Fiction as a subject. Your results may differ.
Both of those titles have a call number starting with 813 and all of your 813s get the subject American fiction. Lew Wallace, author of Ben-Hur, is an American as is Grace Burrowes, the author of Miss Delightful, so that call number makes sense and they should be American fiction.
Use the "International Setting" subject for stories that take place elsewhere. Or some libraries have a variation of "International Setting - Europe" and other regions that might be used as well.
I get it, we are in bed with the AMF subject heading being tied to the 813 call number. This is a case of the letter of the law vs. its spirit. No patron with a subject preference of American Fiction is expecting to have Nightly select either of these titles.