Java Version 8 (build 1.8.0_131-b11)
Some times, after trading with a villager, he seems to "lose" their work station and becomes unavailable. He doesn't become unemployed, while trying to interact with him either nothing happens or the trade window opens and quickly closes again.
If present, an unemployed villager takes over his workplace. The new trader doesn't have the same trading level as the previous.
First time this has happened with a Master Cartographer. It was temporally resolved by placing another work station, but he once again "lost it" and the work station was taken over by an unemployed.
Removing and replacing the workstation did not solve the problem.
In one ocasion an armorer may have abandoned his profession to become a weaponsmith, who had become unavailable (I don't know if that's possible, so he may also have disappeared, got stuck somewhere or died from jumping off an edge).
Confirmed that this has happened with the following traders:
A master cartographer, an apprentice librarian, a weaponsmith, a novice butcher and a novice cleric.
For some time, unavailable traders behave just like other villagers, going to their original workplace, gathering by the bell, sleeping at night, etc etc.
Removing the trader's work station doesn't make him become unemployed.
It seems that if another villager takes over his original work station, the trader is prone to become stuck somewhere. This has happened with all 4 which had their work station taken over. While 3 of them became stuck inside fences, one got stuck in the water.
They still react to players holding emeralds or tradable items by showing items they'd trade for.
The following villagers were traded with and did not become unavailable (so far):
2 farmes (master and apprentice), a master toolsmith, an apprentice fisherman, an apprentice fletcher, a journeyman mason and possibly a novice armorer.
MC-154201 Trying to trade with villager immediately closes trading menu for some villagers