Trading gives you back paid resources in some cases

What is the issue?
When trading sometimes you get back all the paid resources.

How to reproduce
Trade with a villager, e.g. a farmer who is tier 0 and wants to have for example 12 wheat for 1 emerald. Give him 15 (that means more than 12) wheat and take the emerald, take out the 3 wheat he did not take. (Close the menu.) Trade with him again: Give him 15 wheat again and repeat those steps. When you reach the point where he suddenly does not want more wheat, you can take the last emerald he offered. As you want to take out the 3 leftover wheat, you suddenly get back 15 wheat.