Hey, I know this bug often was reported, and often marked as duplicated. I tried to use the workaround, but 1., my custom trades are away, and 2., it does'nt work better...

 - And I tried it with summon a villager in 1.14.2 and 1.14.3, and then open it in 1.14.3. It closes.

The custom villager trade menu close it by its self. When you put a "buy-item" in your hand. Then the villager shows the "sell-items" in his hand. 

What I expected to happen was...:
The menu keep open until I closed.

What actually happened was...:
The menu open for very short time and closed than.

Steps to Reproduce:
1. Summen a villager with a costum trade.
2. Try to trade with the villager.

( I copied this text because my english is very bad  )

Maybe you can fix it.

~TomatoCake