The bug

Clicking a villager's first listed trade while the items for a different trade are already in the trading slot always causes the villager to play its 'agree' sound, even if the player does not have the items required for the first listed trade.

How to reproduce

1. Place some stonecutters
2. Spawn some villagers
3. Give yourself 64 emeralds
4. Find a mason whose first listed trade is clay for emeralds
5. Click the emeralds for bricks trade
   → The villager plays its 'agree' sound
6. After the sound finishes, click the clay for emeralds trade
   → The villager plays its 'agree' sound

Expected result

The villager should play its 'disagree' sound at the final step. This is the observed result if the two trades are listed in the other order.