I first saw this bug in a modded instance of the game with a modded villager trade and tried to repeat this in vanilla minecraft and the glitch is still active on vanilla trades.
I doubt MultiMC altered my vanilla game but i don't think it's the case.

This bug works in both Creative and Survival mod. This is do not affect the actual game mechanics but is 100% visual.

*_STEPS :_*

• Find a villager
• Give the villager more items than he could trade with you (there is a limit of occurrence you can trade with them, you must be able to go over this limit to perform this bug)
• SHIFT+Clic the villager's output
• Close your inventory

*_BUG DESCRIPTION :_*

• The surplus of items spend that goes over the trade limit is dropped away (this may be normal)
• You have visually the items you would receive from trading all your spend items
• If you click the surplus received items, they disappear on the instant.