How to Pre Filter Metabox WYSIWYG post_content

Hi there,
I have a custom post type of 'Testimonials'. For this custom post type I have a MB WYSIWYG field like this:

$meta_boxes[] = [

        'title'   => __( 'Testimonial content', 'text_domain' ),
        'post_types' => 'Testimonials',
        'id'      => 'testimonial_details',
        'context' => 'normal',
        'fields'  => [
            [
		'id'       => 'post_content', //I.E the_content()
                'type'     => 'wysiwyg',
                'desc'       => __( 'Enter your testimonial content.', 'text_domain' ),
                'options'  => [
                    'tinymce'          => [
                        'toolbar1'      => 'formatselect,bold,italic,bullist,numlist,link,|,undo redo',
                        'toolbar2'      => '',
                        'block_formats' => 'Paragraph=p;',
                    ],
                    'drag_drop_upload' => false,
                    'quicktags'        => true,
                    'media_buttons'    => false,
                    'textarea_rows'    => 10,
                ],
                'required' => true,
            ],
        ],
    ];

All works exactly as expected.

The Problem
============

I use a 'content_save_pre' filter function on 'the_content()' for all my post types (Page, Post and CPT) to strip any unwanted tags in the TinyMce Editors like this before it hits the database:

function stripUnwanted($the_content ){

	// Unacceptable tags
	$strip_tags = "div|h1|h3|h4|h5";

	// Filter the_content() on selected post types
	if ( get_post_type( get_the_ID() ) == 'post' || 
         get_post_type( get_the_ID() ) == 'page' || 
         get_post_type( get_the_ID() ) == 'testimonials' ) {  
	    $the_content = preg_replace("#<\s*\/?(".$strip_tags.")\s*[^>]*?>#im", '', $the_content);
	} 
	return $the_content;
}
add_filter('content_save_pre','stripUnwanted',99);

Q) This filter successfully strips my unwanted tags from the built in WordPress TinyMce WYSIWYG wp-editors for 'Post' and 'Page'. However it NEVER catches (filters) the 'testimonials' CPT WYSIWYG generated by Metabox? What am I missing?

Any help / advice appreciated?

Many Thanks 🙂