I expected villagers to reset their trades often enough to avoid ever locking out all trades, but it's possible for a villager to lock out every trade without resetting, making it impossible for the player to ever again trade with that villager.

I'm not talking about only locking out all "buy" trades, but all trades, both buying and selling. This makes the villager useless since there is no way to ever reset the trades.

This has been especially noticed with the villagers who have few possible trades, such as the string-buyer.