Books become a new stack when disenchanting on a grindstone.

After placing enchanted book on grindstone and collecting the result, it becomes a new stack of book with an NBT tag which cannot be stacked to regular books.

However, when shift+clicking the tagged books into an enchanting table and collecting the book again without enchanting, the book gets the tag removed, and is able to be stacked with the regular unenchanted books again. 

This is a low priority bug, as it's only a very minor annoyance.

I hope this was of help.