There is a bug where sometimes with player assisted villages that have a large population where villager will loose their jobs but keep them at the same time. What is happening is a unemployed villager is taking a job station from a villager that was already employed at that station. This results in the umemployed becoming employed and functioning normally BUT the previously employed villager at that station is employed but you cannot trade with the villager. The gui opens for a split second but closes because it is unable to "access" its station. I have personally experienced this happening with a fletcher and toolsmith. I believe distance between bed and station effect this bug because I have 4 lecterns close by the main populated area where most beds and villagers rest and none of them so far have bugged out this way.