If you have stackable items with different components (custom_data, or custom_model_data) in you inventory. The stacks will merge when you select a villager trade that accepts the base item.

Reproductions:

• Spawn a librarian villager.

• Get emeralds and two books a vanilla one without custom components and one with
  • /give @p minecraft:book[minecraft:custom_data=\{Test:1b}]

• Select a trade for an echanted book
• The two books will be merged into one stack