Workflow Messages are Empty when Posting to Mobile App Private Channel

nevilk ,we changed the card JSON format as below and it started rendering properly in both Teams Mobile Public and private channels. Could you please try once and let us know if it renders at your end as well?

{
  "type": "message",
  "attachments": [
    {
      "contentType": "application/vnd.microsoft.card.adaptive",
      "content": {
        "type": "AdaptiveCard",
		"$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
        "version": "1.4",
        "body": [
          {
            "type": "Container",
            "style": "warning",
            "bleed": true,
            "items": [
              {
                "type": "TextBlock",
                "size": "medium",
                "weight": "bolder",
                "text": "Test title",
                "style": "heading",
                "wrap": true
              }
            ]
          },
          {
            "type": "TextBlock",
            "size": null,
            "weight": null,
            "text": "Test description",
            "style": null,
            "wrap": true
          },
          {
            "type": "FactSet",
            "facts": [
              { "title": "custom:stringKey", "value": "value-string" },
              { "title": "custom:stringSetSingleton", "value": "singleton" },
              {
                "title": "custom:stringSet",
                "value": "forth item, third item, tenth item, sixth item, eighth item, eleventh item, eighteenth item, second item, twentieth item, ninth item, seventeenth item, first item, nineteenth item, seventh item, fo..."
              }
            ]
          }
        ],
        "actions": [
          {
            "type": "Action.OpenUrl",
            "title": "View Item",
            "url": "https://example.com"
          }
        ]
      }
    }
  ]
}

Result: (Both in Private and Public channel in Mobile devices)
Android:

iOS: