I had a librarian villager selling mending, and then it reset its trade to Efficiency 5 for absoloutely no reason. Did not break its bed or job site. The villager is in a house that I made. 

How to reproduce:

1. Get a librarian villager. 

2. Break and replace its job site once.

3. Wait ~1 ingame day outside of the village. 

Expected results: Villager has same trades as before. 

Observed results: Villager resets trades.

Workaround: Trade with the villager after you get the trade you want.