Hey,
a GameProfile contains a "SKIN" entry that mostly just holds a url-entry but can additionally carry a "metadata" entry if the skin is marked as "slim".

The default order of these two key-value entries is first "metadata" then "url". By simply changing the order to "url", "metadata" the Minecraft client is no longer able to render the playerskin.

IMO this points to a faulty Json implementation on the client-side as Json Key-Value pairs should not be order dependent.

Example working:

{
  "SKIN": {
    "metadata": {
      "model": "slim"
    },
    "url": "http://textures.minecraft.net/texture/..."
  }
}

Example not working:

{
  "SKIN": {
    "url": "http://textures.minecraft.net/texture/...",
    "metadata": {
      "model": "slim"
    }
  }
}