This has happened since the trading changes occured: When you trade with a villager to his last offers, then buy out one trade, it will say that trade is crossed out, and to use another trade to unlock it. When you exit the trade menu, wait a few seconds, you see particles appear, and the trade is back without having to perform another trade to reopen it.

How to reproduce

1. Spawn a villager in creative mode (or find one)
2. Perform all the trades, and unlock all the tiers
3. Once all tiers are unlocked, perform a single trade (doesn't matter which tier) until it locks
4. Exit the trade menu, and wait until you see purple particles coming from the villager
5. Go back into the trade menu, and notice that the trade is no longer locked, despite not performing another trade