Villagers who have a profession should lose it if they go to the nether consistently. The first time it's pretty reliable, but later they never become jobless, even if I never traded with them. Sometimes it takes longer for them to lose their trade, and sometimes it seems to never happen, especially as the process is repeated.

They often will return to the overworld with their profession/trade intact, again, even if I never traded with them.

It should be an instant change.