Custom Slug cache page refresh all time

I am using the Whatsapp user agent to create a selected cache page. But whenever you access the page it updates the cache. Every time I update the page a new cache is created with a new date. See my code that is in the mu-plugins:

	if ( function_exists( "add_filter" ) ) {
		add_filter( 'supercache_filename_str', 'supercache_custom_ua_keys' ); 
	} else {
		add_cacheaction( 'supercache_filename_str', 'supercache_custom_ua_keys' );
	
		
	}

function supercache_custom_ua_keys($key) {

if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'whatsapp') !== false ){

		$key .= '-whatsapp';				
}

	return $key;
}