Sometimes, when attempting to trade with a villager, the trade goes through for a moment, but then you are refunded whatever you traded and cannot make the trade.

For example: If a villager is trading 4 salmon for an emerald, when I give the villager 4 salmon, the villager will give me the emerald for a moment, but then the trade will be reversed leaving me with the 4 salmon again. Anytime this issue occurs it can likely be replicated again and again afterwards.

The attached video shows this bug in action on 1.14.

My personal speculation is that it that this bug occurs when items are discounted. There was another transaction that I tried completing before on a discounted item that did not work as well. I do not have a video of that though.