What Happened:
If you trade with a villager and destroy the job block, they will not change their job 1
Do it yourself:
1. Take a villager
2. give him a job like Smith
3. trade with him
4. destroy the job block
5. he will not change his job again
Comments 2
Works as intended - check the wiki: https://minecraft.gamepedia.com/Villager#Professions
When you trade a villager you are locking his trades and job, so it's not a bug