It is a bug. Please report it.

In the meantime you could quickly, and simply, set up a workaround. Remove the _required, then use validation by means of jQuery.

You may use this jQuery validation example​:

<form method="post" name="userForm">

  First name:<br>

  <input type="text" name="firstname">

  <br>

  Last name:<br>

  <input type="text" name="lastname" >

  <br>

E-mail:<br>

  <input type="text" name="email">

  <br><br>

  <input name="sbmt" type="submit" value="Submit">

</form>

<script src="https://cdn.jsdelivr.net/jquery/1.12.4/jquery.min.js"></script>

<script src="https://cdn.jsdelivr.net/jquery.validation/1.16.0/jquery.validate.min.js"></script>        

<script type="text/javascript" >

// Wait for the DOM to be ready

$(function() {

  // Initialize form validation on the registration form.

  // It has the name attribute "registration"

  $("form[name='userForm']").validate({

    // Specify validation rules

    rules: {

      // The key name on the left side is the name attribute

      // of an input field. Validation rules are defined

      // on the right side

      firstname: "required",

      lastname: "required",

      email: {

        required: true,

        // Specify that email should be validated

        // by the built-in "email" rule

        email: true

      }

    },

    // Specify validation error messages

    messages: {

      firstname: "Please enter your firstname",

      lastname: "Please enter your lastname",

      email: "Please enter a valid email address"

    },

    // Make sure the form is submitted to the destination defined

    // in the "action" attribute of the form when valid

    submitHandler: function(form) {

      form.submit();

    }

  });

});

</script>