(1.21.5) using item entity display with custom_model_data to show a block model shows up too dark

Trying to make a custom sapling, using item displays because that seems to be the only way to display custom block models without overriding existing blocks; here's how I spawn it:
/summon minecraft:item_display 43 -59.5 30 {item:{id:"minecraft:acacia_sapling",Count:1,components:{custom_model_data:{strings:['3D']}}}}

And here's assets/minecrafts/item/acacia_sapling.json:
{
  "model": {
    "type": "select",
    "property": "custom_model_data",
    "fallback": {
      "type": "model",
      "model": "item/acacia_sapling"
    },
    "cases": [
      {
        "when": "3D",
        "model": {
          "type": "model",
          "model": "block/acacia_sapling"
        }
      }
    ]
  }
}

but as title says, the block model looks darker than an actual sapling (see image)
Is this a known issue & can I fix this & how?
I've tried e.g setting brightness:{sky:15,block:15} inside the components nbt tag but that didn't fix, and I tried overriding models/block/acacia_sapling.json with a copy of models/block/cross.json with texture set to acacia_sapling (works) and changing shade to true in the elements, but that wasn't it either (checked that the file was indeed used by changing the texture to oak sapling and back)