1. Craft a new book. It's a book called "Book".
2. Enchant it. It's an enchanted book called "Enchanted Book".
3. Grindstone it. It's an UNenchanted book called "Enchanted Book".
See attachments for a visual explanation.
Seems to be caused by the fix for MC-148529.
Linked issues
is duplicated by 6
Attachments
Comments 2

The "Enchanted Book"s seem to act normally in every way I've tested except that they don't stack with normal books.
They can be enchanted, used for crafting bookcases, enchanting tables and book and quills and they stack with each other.
Can confirm.