The bug

When trading an item that has different nbt tags than the one the villager requests, it does not auto fill when clicking the trade. If you put the item in however, it does allow you to trade it.

How to reproduce

1. Summon a villager that buys stone:

   /summon villager ~ ~ ~ {VillagerData:{profession:farmer,level:6},Offers:{Recipes:[{buy:{id:stone,Count:1},sell:{id:emerald,Count:1}}]}}
   

2. Ensure you have no stone in your inventory:

   /clear @s stone
   

3. Give yourself a block of stone with an NBT tag:

   /give @s stone{tag:0}
   

4. Open the villager's trading GUI and click on the stone trade
   → The stone is not inserted into the trading input slot
5. Manually place the stone in the trading input slot
   → You are able to complete the trade