Curing stops when re-joining a world

I was curing a zombie villager and when I re-joined my world, the curing has stopped. It's a waste of resources!

What I expected to happen was...:
The curing process would continue after I re-joined my world.

What actually happened was...:
Instead, it stopped when I re-joined my world.

Steps to reproduce:
1. Cure a zombie villager
2. Exit and re-join the world after starting the curing process
3. Notice that it has stopped. Am I supposed to go to the nether again?