Villager Discount not working

I have 33 Melon blocks on me, ready to trade with this villager

• I put all the Melons in the interface, as should be
• I shift click out the emeralds (Melons appear as they should, with the discount)
• Shift click out melons, and i get less than i should, approximately 6 melons left, which is equal to how many emeralds i got.

(He also denies to trade 2 Melons for 1 emerald)