Problems:
1) No experience bar;
2) No experience level displayed in window title;
3) Can be sold infinite amount of items without locking a trade.
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I have encountered the same issues when trading with experienced legacy villagers. Different kinds of legacy villagers exist: (1) pre-1.8 villagers (random trades), (2) 1.8 legacy villagers (fixed trade offers, random prices). The issue appears to affect all of them.
No experience bar - this may be working as intended if the villager is Master level (diamond tier). If the villager is lower level, this is probably a bug.
No experience level - this seems to happen with legacy villagers.
Infinite trades - this won't happen initially. After they restock, trades become infinite.
Are you using a resource pack? If so, try disabling it and see if the issue persists.