After Validation Error, my Input Mask function doesn't work on field

After Validation Error, my Input Mask function doesn't work on field

I am using JQuery to successfully validate my form.

I added an masked input to my birth date field in my form. The masked input plugin is from: https://github.com/RobinHerbots/jquery.inputmask

On load, the birthdate field has the masked input working properly. If the form is submitted and validation is failed on that field of other fields in the form: the birthdate field is stripped of it's masked character separators and the masked input is not working anymore. 

I need a way to keep that mask in the input box if the validation fails.  I am confused as to why the input mask is cleared completely from the input box just because the validation fails!  

Here's my birthdate field from the form:             
  1. <div class="col-md-4">
  <div class="form-group">
    <label class="control-label">Date of Birth</label>
     <div class="input-icon right">
       <i class="fa"></i>
       <input id="birthdate" name="birthdate" type="text" class="form-control">
     </div>
  </div>
</div>
My script: 

  1. <script>
$(document).ready(function() {
          
        var form = $('#add_contact_quick');
        var error = $('.alert-danger', form);
        var success = $('.alert-success', form);

         $('#add_contact_quick').validate({
                errorElement: 'span', //default input error message container
                errorClass: 'help-block help-block-error', // default input error message class
                focusInvalid: true, // do not focus the last invalid input
                ignore: "",  // validate all fields including form hidden input
                rules: {
                    friends_call_me: {
                        required: true
                    },
                    first: {
                        required: true
                    },
                    last: {
                        required: true
                    },
                    email: {
                        email: true
                    },
                    birthdate: {
                        date: true 
                    },
                },

                invalidHandler: function (event, validator) { //display error alert on form submit              
                           success.hide();
                           error.show();
                           Metronic.scrollTo(error, -200);             
                },

                errorPlacement: function (error, element) { // render error placement for each input type
                            var icon = $(element).parent('.input-icon').children('i');
                            icon.removeClass('fa-check').addClass("fa-warning");  
                            icon.attr("data-original-title", error.text()).tooltip({'container': 'body'});
                },


                highlight: function (element) { // hightlight error inputs
                            $(element)
                                .closest('.form-group').removeClass("has-success").addClass('has-error'); // set error class to the control group   
                },

                unhighlight: function (element) { // revert the change done by highlight

                },

                success: function (label, element) {
                    var icon = $(element).parent('.input-icon').children('i');
                    $(element).closest('.form-group').removeClass('has-error').addClass('has-success'); // set success class to the control group
                    icon.removeClass("fa-warning").addClass("fa-check");
                },

                submitHandler: function(form) {
                    //form[0].reset();
                    success.show();
                    error.hide();
                    Metronic.scrollTo(success, -200);
                    $.ajax({
                        url: "ajax_insert.php?table=contacts",
                        type: form.method,
                        data: $(form).serialize(),
                        success: function(response) {
                                //response here if data response
                            if (response.redirect) {
                                // data.redirect contains the string URL to redirect to
                                    window.location.href = reponse.redirect;
                                    }
                        }        
                    });
                    
                }

         }); 
   $("#birthdate").inputmask("99/99/9999");
});
 
</script>
Any thoughts why the input mask stops working after a failed validation? Thanks.