New version .NET 6 is not work without get set

Question

My old project is using .NET Core 2.2 and I need to migrate it to .NET 6.

The previous developer added some classes something like this:

public class ClassName
{
    public int code;
    public string message;
}

And API controller

    [HttpGet]
    [Route("GetTotal")]
    public List<ClassName> GetTotal(DateTime dtCur)
    {
        List<ClassName> l = new List<ClassName>();

        ClassName n = new ClassName()
        {
                code = -99,
                message = "invalid session"
        };

        l.Add(n);

        return l;
    }

It worked on .NET Core 2.2, but on .NET 6, it always returns an empty array from ajax or Postman.

To get it done in .NET 6, I must fix class like this:

public class ClassName
{
    public int code { get; set; }
    public string message { get; set; }
}

But the problem is all classes in old projects they did not add { get; set; } to any classes, this mean I must fix all them.

Is there any way to fix it quickly, or someone can explain why in .NET 6 return empty in this case?