This bug happens when you try to cure a zombie villager and immediately quit the game. To reenact this situation, you just have to do the normal steps to cure a zombie villager but instead of waiting for those 5 minutes or so to finish the curing, exit the game to the menu page. Then, you enter that world again and the villager is instantly cured.

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Edit, 07.07.21:
Copied from MCPE-106839 - a new, confirmed behavior was discovered related to the bug:
Normally, a villager being cured multiple times can offer a heavier trading discount than one being cured once. However, when one save & quit the game while a villager is still being cured, upon returning to the game the villager will forget whether it has been cured before.

Steps to Reproduce:

1. Convert and cure a villager (A) multiple times, and record the trading prices. The prices would drop every time A is cured again.
2. Convert and cure A again, but save and quit the game when A is still in zombie form.
3. Return to the game. A should now be a villager.
4. Check the trading price of A. The price would be as if A is cured the first time, regardless of how many cures have been performed in step 1.