Villagers will randomly lose their job and reselect another workstation. 

Created a trading hall where the villagers were brought in one by one, from more than 100 blocks away, and given a workstation. Attached picture is the setup I have. The villager is in a minecart on the detector rail and the workstation is always on their left, on the stone block behind them. I would place the workstation 3 times to watch them lose the profession to make sure they were attaching to the right one, and I'd verify with the /data get command that the coordinates of the workstation were correct.

Once the trading hall was about halfway full, after traveling to the nether, out of range and therefore unloading the chunk, and overall playing normally, I went back to check the coordinates of their workstation and they had changed to one across the hall and some villagers were unemployed. Their first trades had changed as well. 

I thought maybe that it was minecarts so I spawned them in with eggs and did the same thing and the same result. 

The villagers face 3 different directions so it's likely not directional, and the trading hall goes over multiple chunks. 

There is also a villager breeder a few chunks away, up at the sky limit. 

Working on a world download, it's not the smallest. 

Link to download: https://www.dropbox.com/s/ztvvp913mgtch0m/world.zip?dl=0

Coordinates of screenshot are 349 52 -1193