I experienced this bug with build #129 and #131 of Paper 1.14.3.
The trading GUI is opening for half a second before it closes again.
This did not happen when the villagers first spawned. It happend after I traded multiple times. I think it has nothing to do with the trade level of the villager, since the bug is happening with villagers with different levels.
Maybe it has something to do with the change in the stock items of the villagers.
I also experienced it before I became the saviour of the village and after.
Edit: The bug common occurs with the fletcher.
note: I have only found one other topic which talks about this problem. on github: https://github.com/PaperMC/Paper/issues/1658
Seeing this is from November 2018, tells me this bug also occured before .1.14.3
I have encountered no other bugs so far and I am very pleased with the stability compared to other server tools.
Hopefully this bug will be fixed.