Villagers can't decide whether or not their trades are locked, which causes the player to lose items and prevents them from trading.

Case 1. Background: I was doing regular trading when this issue occured. I had a farmer villager that traded 1 emerald for 5 pumpkins (original price 6). I put in 64 pumpkins and shift-clicked to claim the emeralds. The trade locked, villager took 30 pumpkins and I got 2 emeralds (should've gotten 6). I took out the remaining 34 pumpkins but only got 4 pumpkins and I also lost the 2 emeralds. Result: I got no emeralds but I lost 60 pumpkins (note: this may not have been the end result, it could have changed after I opened my inventory. In any case, I did lose pumpkins).

Note: This issue occured several times but I unfortunately filmed only one of the times and I stopped the recording after I closed the GUI. This description is based only on the one case where I filmed it so I don't know what happened next time I opened my inventory. It is possible that some of the items reappeared in my inventory as in Case 2, but it's for sure that I did lose items in the process.

Case 2. I also experienced a similar issue with the same villager when I was putting in pumpkins and carrots. The trade locked but I didn't get or lose any items. Next time I opened the GUI, the trade was unlocked and the same issue repeated over and over as I tried to trade with it. Result: Wasn't able to trade with villager in a while (seems like it started working again when I traded with the villager at night time).

These issues were experienced on a 1.14.2 vanilla server. I tentatively tried to recreate them in a test world but got no result. There are several factors that could affect this. I also loaded the world in 1.14.3-pre 1 singleplayer. At first the price of the pumpkins had changed to 7, but it soon went back down to 5. I didn't experience the two issues I made this bug report about though.

Image of the trading setup and video of the two issues: