Villager trade GUI doesn't show the correct price on servers if trade demand is high

To reproduce

1. Start a server (or use a LAN world and follow the following steps with a second account)
2. Join the server
3. Spawn a villager zombie
4. Cure it
5. Give it a profession
6. Trade with it until it is completely locked
7. Let it restock
8. The prices should now be slightly increased
   → When trading with the villager, it will take more items from you than displayed in the GUI

Original description

I had a one emerald villager now he still shows that trade but glitches out when i try to trade with him. I was able to get ~6 books out of him and after that i can grab the book out of the menu and then it disappears. Soon after I leave the GUI my emerald/book comes back in my inventory. Same with the same villager's paper trade. Some times he takes my paper, sometimes they just come back to my inv. after i leave gui. 

I think the GUI is just Not updating correctly to what trade the villager has available.

UPDATE: I found that the villager actually knows how much emerald it wants for each item. when i put 19 paper in like the GUI suggested, the emerald shows up on the right, but it disappears if you try to grab it. However if I put around 40 paper in, he will actually give you the emerald. Same with the mending book situation, where he actually wants 2 emerald instead of the 1 listed on GUI.