The bug
The value should increase by 2*(trades done), but when doing the trades using shift-click it only counts is as one trade.
How to reproduce
Summon a villager and place a working site block, e.g. a brewing stand
Check their gossip
/data get entity @e[type=villager,limit=1,sort=nearest] Gossips
✔ It is empty as expected
Trade with them but use the trade multiple times by shift clicking it
Check the gossip of the villager again
/data get entity @e[type=villager,limit=1,sort=nearest] Gossips
❌ The trading entry only has a score of 2
Code analysis
Using Mojang names
Trading gossip is only added every tick by net.minecraft.world.entity.npc.Villager.customServerAiStep()
instead of directly after the trade.
Can confirm in 20w51a.