Because of some of the changes made to villagers within the 1.14.4 pre-releases, villager "demand" nbt will never go down on its own. In previous versions, demand would slowly go down whenever a villager would work at their workstations, regardless of being traded with. Since villager demand only goes down when a villager "restocks" a villager's demand will never go down unless one of its trades is locked out and it restocks its trades.
MC-156042
Villager demand never goes down over time unless traded with
Resolved
Fixed
2
Confirmed
Important
Trading
Minecraft 1.14.4 Pre-Release 2, Minecraft 1.14.4 Pre-Release 3
Minecraft 1.14.4 Pre-Release 5
Retrieved
affects 1.14.4 pre-release 3
I thought I should mention that this really should be addressed as it causes a couple of large issues if you do any sort of villager trading.
Mainly, if you only trade one type of item from a villager, you can never lower the prices for that trade.
Essentially, it means that you could never make a profit from trading with villagers if you were trying to get emeralds in some situations, or it would mean that you could never reset prices back to their original costs as there would always be a trade with high demand on a villager.