First reported by Motherboard here: https://motherboard.vice.com/en_us/article/bjpjn4/amazon-erotica-best-seller-rankings-removed
with more information & updates in the URL above
Apparently Amazon (in its infinite wisdom about books -- enough that they can't tell the Bronte sisters apart, let alone their novels) is deciding on its own what is "erotica" and what is "romance" - and they're banning/not listing/not counting/not advertising various titles that they have decided fall into the erotica part of that equation. I just read "erotica" during an historical fiction book, so I do wonder how they're making the decisions.