Issue with block model tinting and overlay textures

[Sheepy3]

Description

hello, having trouble with getting tint functionality working on a block. Specifically, I want to tint a specific part of a block using an overlay, using the grass block model as reference. my model JSON is this:

{
  "parent": "minecraft:block/block",
  "textures": {
    "bottom": "sheepymod:block/thatch_block_top",
    "top": "sheepymod:block/thatch_block_top",
    "side": "sheepymod:block/thatch_block_tocolor",
    "overlay": "sheepymod:block/thatch_block_top_mask"
  },
  "elements": [
    {   "from": [ 0, 0, 0 ],
      "to": [ 16, 16, 16 ],
      "faces": {
        "down":  { "uv": [ 0, 0, 16, 16 ], "texture": "#bottom", "cullface": "down","tintindex": -1   },
        "up":    { "uv": [ 0, 0, 16, 16 ], "texture": "#top",    "cullface": "up","tintindex": -1  },
        "north": { "uv": [ 0, 0, 16, 16 ], "texture": "#side",   "cullface": "north","tintindex": 0  },
        "south": { "uv": [ 0, 0, 16, 16 ], "texture": "#side",   "cullface": "south","tintindex": 0  },
        "west":  { "uv": [ 0, 0, 16, 16 ], "texture": "#side",   "cullface": "west","tintindex": 0  },
        "east":  { "uv": [ 0, 0, 16, 16 ], "texture": "#side",   "cullface": "east","tintindex": 0  }
      }
    },
    {   "from": [ 0, 0, 0 ],
      "to": [ 16, 16, 16 ],
      "faces": {
        "up": { "uv": [ 0, 0, 16, 16 ], "texture": "#overlay", "tintindex": 0, "cullface": "up" },
        "down": { "uv": [ 0, 0, 16, 16 ], "texture": "#overlay", "tintindex": 0, "cullface": "down" }
      }
    }
  ]
}

however, this model results in the entire top of the model being the color of the tint, rather than just the pixels of the overlay (you can see something closer to the expected result in my hand in the image below). I assume I'm missing something important, but I'm not sure what.