Books no longer showing as "books"

I just got some update to my Kindle Fire HDX 7. It's not the OS5, but I guess some other update, looks like OS 4.5.5.1

All of a sudden, anything sideloaded doesn't get put under books. I've been doing this for I don't know how many years, and I've never had this happen before. They are all now appearing in DOCS instead. It's quite annoying.

Any thoughts?

Thanks!