Basically whenever you try and trade with villagers it shows that it worked, but once you exit the GUI it reverts the trade as if nothing happens. Here is a video of it happening: https://imgur.com/pxZT5Xv
Also tested this on a vanilla server and it worked perfectly fine on there so this is something to do with Spigot/Craftbukkit.
- relates to
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SPIGOT-4913 Villagers not choosing professions
- Resolved