Summary: In a trading hall setup, where villagers in individual 1x2 (and possibly 1x1) cells have bonded to a workbench adjacent to them, they will sometimes un-bond from the workbench. Being trapped in their cell next to the workbench, they will stop unlocking their trades off their previously-bonded workbench. Other nearby unemployed villagers can also then bond to the workbench.

Also see the YouTube video I made about this: https://youtu.be/Qpw5ub2TR_A

To Reproduce: (I hope you can reproduce. I've heard several people report this from my trading hall tutorial and have reproduced it myself.)

1. Place a villager in a 2x1 cell surrounded by solid blocks, with 1-wide by 2-tall opening with a workbench on the bottom half of the opening and a trapdoor, slab, or nothing on top of the workbench. I've tried this with a lot of variations and they all enable the bug. [See screenshots of the villager holding cells and the second villager in a minecart nearby who has picked up the workstation after the first un-bonded from it]. 
2. Make sure they have bonded to the workbench. This bug should manifest whether you advance their profession past Novice and they have refreshed their trades a few times, or whether they have never been traded with.
3. Place an unemployed villager outside the cell, but still able to path-find to the workbench the initial villager has bonded to. The villager can also be in a minecart. This second villager is not required, but helps illustrate the issue.
4. Keep advancing through several days of Minecraft time using villager-related times of day. [Time Set 0, Time Set 2000, Time Set 6000, Time Set 9000, Time Set Night] The un-bonding can happen after a few days or even up to 20 days, in my experience.
5. Observe that eventually the villager in the cell will un-bond from its workbench, and the new unemployed villager will suddenly become the profession type associated with that workbench. This other villager will also walk over and work at that workbench, which the original villager in the cell will be unable to work at it or unlock their trades. If the villager in the cell was still a Novice, they will become unemployed. [See attached video file]
6. If the first villager was a Novice, kill the the second villager that has bonding to its old workbench. Watch the first villager try to re-bond to the workstation, and you may observer the "flickering" issue as it tries to pick up its workstation again. [See second video file attachment]

Implications: This bug may break any sort of traditional organized trading hall setup. Even an open room full of workbenches and wandering villagers could result in all of them eventually un-bonding and failing to unlock trades.

Reference: Please see the video file I've attached to this report. This bug has been observed by me, and also reported by several people in the trading hall setup from this tutorial: https://www.youtube.com/watch?v=Lk3yLDCDdRA Note that after 1.14.0, the hall was modified to place a slab or trapdoor over the workbench instead of between the villager and workbench, to allow open path-finding that the villagers now require. See also the screenshots.