My custom meta box with textarea field not saved

sanitize_textarea_field() will be stripping out the GMaps <iframe> element. Sanitize it with wp_kses( $store_map, $allowed ) instead, where $allowed is an array in the following format:

$allowed = array(
    'iframe' => array(
        'width' => array(),
        'height' => array(),
        'style' => array(),
        'loading' => array(),
        'allowfullscreen' => array(),
        'referrerpolicy' => array(),
        'src' => array()
    )
);

listing each attribute you specifically want to allow (I took mine from the example here).