Villagers do not update to the level system if they have all trades discovered

How to reproduce:

1. Create a world in 1.13.2.
2. Find/Spawn two librarian villagers.
3. Trade with one of them until you discover its entire set of trades (until you reach the name tag).
4. Trade with the other one until you unlock a trade for the first time. Do not unlock all of its trades.
5. Load the world in 1.14.
6. Look at the trading screen of the villagers. The one wich had all trades discovered will not have a level/title, while the one who did not had all its trades unlocked will have a level/title.
7. Trade until you unlock all of the remaining trades of the villager who still had hidden offers.
8. Compare the two villagers. One will have "Librarian" as a title and the other one will have "Librarian - Master".

This will work in singleplayer and multiplayer. I have succesfully tested farmers, librarians and cartographers. All professions seem to have the bug.

Expected behavior

"Librarian - Master" instead of "Librarian".

Actual behavior

"Librarian" instead of "Librarian - <Level>".