Are there any restrictions on the JSON fields for the adaptive cards, type messageBack

Hi! I'm using an adaptive card with content buttons, sending a button with type "messageBack" (I also tried "postBack"), and a JSON set as value. All good, but it looks like some keys get stripped from my JSON value.

For example, I'm sending 

{
  "text": "state?",
  "attachmentLayout": "list",
  "attachments": [
    {
      "contentType": "application/vnd.microsoft.card.hero",
      "content": {
        "buttons": [
          {
            "title": "state 1",
            "type": "messageBack",
            "value": "{\"state\":111,\"test2\":2222,\"test3\":333}"
          }
        ]
      }
    }
  ]
}

And when the button gets clicked, I get back as value

{
  test2: 2222,
  test3: 333,
}

Looks like the "state" key from my JSON got removed. Any ideea why? Or maybe there's a link in the documentation with some keywords? That can't be used? Or I'm doing something wrong here.

Thanks!