WordPress custom content area with wp editor?

Using my Wordpress theme features file, I created a custom meta text area (see screenshot below).

When editing a page, the custom element text area is displayed.

Both saving and changing the text work fine, but if I change a specific part of the paragraph text to heading2 and click save, it reverts the heading2 to the paragraph.

After my function update the meta box looks like this:

if( isset( $_POST['my_meta_box_text_challengetbl'] ) )
    update_post_meta( $post_id, 'my_meta_box_text_challengetbl', wp_kses( $_POST['my_meta_box_text_challengetbl'], $allowed ) );
}

The php code for $allowed looks like this:

$allowed = array( 
    'a' => array( // on allow a tags
        'href' => array() // and those anchords can only have href attribute
    )
);