Cured villagers raising their prices

Hello, I'm experiencing an issue with my villagers raising their prices after being cured. Here's what I've done: I placed a villager in a hole with a blast furnace and infected him with a zombie. After curing him, his prices were lowered, and I was pleased with the results. I then went mining for about 20 minutes. However, when I returned, I noticed that his prices had increased from 32 6, to 32 20.

I waited for about an hour, but the prices did not shift at all. Just to clarify, I did not trade with him at all after the curing process. Has anyone else experienced this issue? Please let me know if you have any insights.