No validation in Ajax front-end submission

Some additional observations.
Safari, Chrome, WP 5.3.2, TwentyTwenty theme

When I defined the fields in PHP instead of MB Builder with 'validation' attribute like below:

     'validation' => array(
            'rules'  => array(
                $prefix . 'first-name' => array(
                    'required'  => 1,
                ),
                $prefix . 'last-name' => array(
                    'required'  => 1,
                ),
            ),
        )

1. Validation errors are properly visible in non-Ajax submit
2. Validation errors are replaced with success message in Ajax submit
3. In both cases the fields labels have duplicated asterisk marking required fields "* *"
4. When I don't use 'validation' attribute and rely just on browser's validation, the submit field remains "disabled" when I fill the fields after initial submit attempt and cannot be re-submitted (non-Ajax form)

I'm sad a bit as I wanted to use all this stuff on production without installing third-party plugins...

Hello, thank you for taking a look at that.

So far I'm using a workaround to keep MB Builder and validations together.

1. I disabled Ajax for front-end forms
2. I added a filter which modifies existing meta boxes by inserting validation formulas

add_filter( 'rwmb_meta_boxes', 'prefix_meta_boxes_validation_rules', 999 );
/**
 * Add validation rules to existing meta boxes.
 *
 * Filter hook: 'rwmb_meta_boxes'
 *
 * @since 1.0.0
 *
 * @param array $meta_boxes Meta Box plugin's meta boxes.
 */
function prefix_meta_boxes_validation_rules( $meta_boxes ) {

    foreach ( $meta_boxes as $i => $meta_box ) :
        $required = [];
        foreach ( $meta_box['fields'] as $field ) :
            if ( isset( $field['required'] ) && $field['required'] ) :
                $required[] = [ $field['id'] => [ 'required' => 1 ] ];
            endif;
        endforeach;
        if ( ! empty( $required ) ) :
            $meta_boxes[$i]['validation'] = [ 'rules' => $required ];
        endif;
    endforeach;

    return $meta_boxes;
}