New to adaptive cards - this renders fine on desktop when sent via a webhook but just shows up blank on iOS. Am I missing something obvious? Thank you

{
    "type": "message",
    "attachments": [
        {
            "contentType": "application/vnd.microsoft.card.adaptive",
            "content": {
                "type": "AdaptiveCard",
                "body": [
                    {
                        "type": "TextBlock",
                        "size": "extraLarge",
                        "weight": "bolder",
                        "text": "Test"
                    },
                    {
                        "type": "FactSet",
                        "facts": [
                            {
                                "title": "Test 1",
                                "value": "Test 1"
                            },
                            {
                                "title": "Test 2",
                                "value": "Test 2"
                            }
                        ]
                    }
                ]
            },
            "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
            "version": "1.4"
        }
    ]
}