Occasionally you will trade with a villager, doing all its trades. There is a significant chance that all the villager's trades become locked with no way to unlock them. This is especially common in all the 1.14 snapshots with all the changes to villager trades.
Edit: I just found a similar issue, though I should add that trades will also become all locked on a diamond-tier villager:
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Comments 3
I can confirm this, I traded with an armorer (tier 0), gave him all the coal and bought all of his iron helmets, both trade possibilities got locked, but he never unlocked the next tier. Nothing more to trade with him. 😞
Duplicate of MC-141887.
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