ModelState stays valid even if the required attributes are empty in the .NET Framework Web API 4.7.2

I have a strange situation, and none of the existing solutions help

I have this model in the .NET Framework Web API

public partial class RequestData
{
    public int Id { get; set; }
    [Required(AllowEmptyStrings = false)]
    public string AccountName { get; set; }
    [Required(AllowEmptyStrings = false)]
    public string VendorName { get; set; }
    [Required]
    public string Memo { get; set; }
    public Nullable<bool> IsToBePrinted { get; set; }
    [Required]
    public string AccountType { get; set; }
    [Required]
    public Nullable<double> Amount { get; set; }
    public Nullable<bool> IsProcessed { get; set; }
    public string Ticket { get; set; }
    public string Response { get; set; }
    public Nullable<System.DateTime> Updated { get; set; }
    [Required]
    public Nullable<int> RequestType { get; set; }
    [Required]
    public string TxnId { get; set; }
    [Required]
    public string EditSequence { get; set; }
    [Required]
    public string ListID { get; set; }
    [Required]
    public string TxnLineId { get; set; }
}

I have a POST controller that uses this model

    [HttpPost]
    public ActionResult PostNewRequest([FromBody] RequestData requestData)
    {
        try
        {

            if (!ModelState.IsValid) {
                //Your code if the field is invalid
            } else {
                //Your code if the field is valid
            }

I also tried to use the filtering functionality but it is not working

    public class ValidateModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.ModelState.IsValid == false) {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, actionContext.ModelState);
            }
        }
    }