What I expected to happen was...:

Opened and unopened books are stackable

What actually happened was...:

Opened written books cannot be stacked with unopened books.

Steps to Reproduce:
1. Write something in a book and quill and sign it
2. Use the middle mouse button to get an identical copy of the book
3. Note the books are stackable
4. Hold one single book in your hand and open it
5. Try to stack the opened book with the other unopened copy you created in step 2
6. Note that the books can no longer be stacked

This is caused by the same problem as described in MC-224343