Steps to reproduce:

1. Spawn a villager in a single-player world
2. Give the villager a jobsite block so it can be traded with
3. Hit the villager to increase its prices
4. Check if the prices for that villager has increased
5. Save and quit to title, and relaunch Minecraft
6.  The villager no longer has increased prices

In addition, other mob behaviors are also reset when Minecraft is restarted. For example, 

1. Foxes no longer trust the player that bred them
2. Endermen and bees are no longer angry at the player that angered them. (This may be intentional as the server in a single-player world restarts when the player saves and quits to the title screen). 
3. Tamed dogs and cats that are sitting stay sitting, even when the player that tamed them right-clicks on them.
4. Villagers also no longer give discounts to players that unlocked their master-level trades or cured them. 
5. The skin for the player changes randomly from Alex to Steve and back.

This behavior also occurs in the 1.16 snapshots (20w06a to 20w30a), the 1.16 pre-releases (for both 1.16, 1.16.1, and 1.16.2), the 1.16 release candidates (for both 1.16, 1.16.1, 1.16.2, and 1.16.3), and the full releases themselves.

This bug is extremely similar to MC-199784, which explains that the UUID for the Player resets each time the world is restarted, causing the aforementioned behaviors. In addition, this bug is also similar to MC-190552, which states that the inventory and position resets each time the world is restarted, which can also be explained by an UUID reset. 

Sorry if this bug is a duplicate of the aforementioned bug.