Newtonsoft JSON.NET ignores default values in parametrized constructor when deserializing

video = "This is a video label" is a C# call-site default, not a deserialization fallback. C# uses that value only when the caller omits the argument; in your debug session Json.NET is supplying "", so the constructor has no missing argument for C# to fill.

Put the JSON-specific defaulting in the constructor body:

public class MyClass
{
    private const string DefaultVideoLabel = "This is a video label";
    public readonly string ID;
    public readonly string VideoLabel;
    public MyClass(string id, string video = DefaultVideoLabel)
    {
        ID = id;
        VideoLabel = string.IsNullOrEmpty(video) ? DefaultVideoLabel : video;
    }
}

With this code, both a missing value that arrives as null and the "" value you are seeing become the default label.

If "" is a valid explicit value in your JSON and only missing/null should use the default, change the assignment to:

VideoLabel = video ?? DefaultVideoLabel;

DefaultValueHandling.Populate does not fix this constructor-parameter case; it does not make Json.NET omit the video argument so that C#'s optional-parameter default is applied.