I was trying to get a woodland explorer map from a cartographer. I traded with it to unlock the trade and get emeralds. I unlocked the trade, but was missing a piece of iron for the compass. I went and got it, came back, ant the cartographer’s trades had reset, even after I traded with it. This was very annoying as I worked hard to unlock the trade. It went straight back to novice. 
1. trade with cartographer villager until level 3 is unlocked 

2.leave for 10 minutes 

3. come back to villager 

The trades should’ve stayed the same, but they reset completely.