Some villagers from previous versions, including 1.14.3 pre-release-1, behave as if they don't have a profession. Oddly, their old trades are still visible for a split second upon right click, but the UI closes immediately.
Related issues
duplicates
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migrated
migrated
This is caused when a villager that has been traded with loses it's workstation and placing back the workstation will let you trade as normal.
migrated
That can't be true because I haven't broken any villager work stations since a villager was assigned to them.
violine1101
Duplicate of MC-154201
Getting the same bug. Only getting it with Villagers with experience in their professions, though. 0xp novices work fine.