This is a vanilla bug I believe, found on a realms server. Enchanted books that you put in the grindstone to unenchant do not restack on books that have not been through the grindstone.
Linked issues
duplicates 1
Comments 1
MC-149687
Vanilla book stacking issue
Resolved
Duplicate
0
scae burge
(Unassigned)
Unconfirmed
(Unassigned)
Minecraft 1.14
Retrieved
Duplicate of MC-148476