Fletcher can trade 2nd trade on first trade

Fletcher will trade his 2nd/[3rd]/4th? trade if you place emeralds in the first trade window, In the second trade slot.

What I expected to happen was...:
Nothing should happen I shouldnt be able to trade like that

What actually happened was...:
he traded me arrows when there was string for the trade

Steps to Reproduce:
1. Open UI on fletcher villager
2. Place emeralds in 2nd [user] trade slot
3. Fletcher trades his second trade