In vanilla, I can grind 2 curse books into 1 item of 2 copies.

However, when this mod enabled, this feature disappears.

Interesting, I didn't even know that feature existed (it doesn't really make sense either, because items with NBT data usually can't stack). I'll try to fix this.

it's quite an amazing feature that youtuber cubicmetre has found. It can be used in item sorter machine, which makes the machine stock with only 1 expected item to fit in.
https://youtu.be/Gyo-dYoW09s?t=318

I've found the cause: when two enchanted books are in the grindstone's slots, the "move enchantments" feature tries to move the first enchantment of the first book to the second one. However, that only works if the book has at least two non-curse enchantments.
I've now changed it to keep the vanilla behaviour in this case instead of blocking the output slot (in version 1.6.4).