The bug

Villagers showing the item they want to trade ignore the NBT data, even though it is checked when trading.
This means a villager can indicate that it wants to trade by holding an item even though the trade is not possible.

Just to be clear, I don't want/expect that non-matching custom tags can still be traded with, I just want the villager not to show the item being sold in it's hands if an item with non-matching tags is held.

How to reproduce

1. /summon villager ~ ~ ~ {VillagerData:{profession:"minecraft:nitwit"},Offers:{Recipes:[{maxUses:100,buyB:{id:stone,Count:1b},buy:{id:stone,Count:1b},sell:{id:stone,Count:1b}}]}}
   

2. /give @s stone{display:{Name:"[\"Test\"]"}}
   

3. Hold the stone item in your hand
   The villager shows a stone item indicating that they want to trade
4. Try to trade
   It does not work (as expected) because the NBT of the items does not match