I've created an achievement that activates when you interact with a brown mooshroom with a bowl in hand.

However, it does not allow you to use the stew_effects NBT as a condition; in that case the advancement is never granted.

To reproduce

1. Add the attached data pack mc-274126.zip to your world
2. Summon a brown mooshroom:

   /summon minecraft:mooshroom ~ ~ ~ {Type:"brown"}

3. Use a bowl on the brown mooshroom
   → The player receives the "lightning-fueled snack" advancement (bugtest:mooshroom/works.json)
4. Feed the brown mooshroom a wither rose
5. Use a bowl on the brown mooshroom again

Expected behaviour

The player receives the "withery stew" advancement (bugtest:mooshroom/doesnt_work.json) since it is identical to the previous one, except that it requires the following NBT to be present on the mooshroom:

{stew_effects:[{id:'minecraft:wither'}]}

Observed behaviour

No advancement is given.

Advancement definition

This is the criterion for the advancement that does not work:

{
  "criteria": {
    "requirement": {
      "trigger": "minecraft:player_interacted_with_entity",
      "conditions": {
        "item": {
          "items": "minecraft:bowl"
        },
        "entity": {
          "type": "minecraft:mooshroom",
          "type_specific": {
            "type": "minecraft:mooshroom",
            "variant": "brown"
          },
          "nbt": "{stew_effects:[{id:\"minecraft:wither\"}]}" // remove this, and it works!
        }
      }
    }
  }
}

Notes

Other NBT fields work properly, and the stew_effects NBT field is properly saved and reloaded when closing and rejoining the world.