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

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"    
}

Start with the supplied Card JSON and the Designer's parsing path for the lang property, then try to reproduce the error after editing and reverting the JSON. Done means valid ISO-639 values such as "en" parse consistently without the reported error.