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

1. Summon a villager and place a working site block, e.g. a brewing stand
2. Check their gossip

   /data get entity @e[type=villager,limit=1,sort=nearest] Gossips
   

   It is empty as expected

3. Trade with them but use the trade multiple times by shift clicking it
4. 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.