[Designer] Setting a language sometimes results in a parse error

[sabberworm]

Problem Description

Setting the lang property to an ISO-639 language code sometimes shows an error:

[Parse] Invalid value "en" for property "lang".

The error doesn’t trigger reproducibly. It mostly happens when I manually edit the JSON slightly (even if I revert the edit afterwards).

Card JSON

{
    "type": "AdaptiveCard",
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.6",
    "body": [
        {
            "type": "TextBlock",
            "text": "Test",
            "wrap": true
        }
    ],
    "lang": "en"    
}