使用WordPress最终输出过滤器缓存问题

I'm using buffer method for final html output recommended by kfriend here https://stackoverflow.com/a/22818089/3465265

It works fine but have issues with cache plugins. Arne L recommended a filter for WP Super Cache here: https://stackoverflow.com/a/51171913/3465265

But that's not working. When i use wp_cache_ob_callback_filter as filter name, there are no changes at all. No super cache files are generated.

WP Rocket also have similar filter called rocket_buffer https://docs.wp-rocket.me/article/774-rocketbuffer

Applying that filter name fix minifying issue. HTML and CSS both are minified properly by WP Rocket but no html files are generated inside /cache/wp-rocket directory.

Here is my code:

ob_start();

add_action('shutdown', function() {
    $final = '';

    // We'll need to get the number of ob levels we're in, so that we can iterate over each, collecting
    // that buffer's output into the final output.
    $levels = ob_get_level();

    for ($i = 0; $i < $levels; $i++) {
        $final .= ob_get_clean();
    }

    // Apply any filters to the final output
    echo apply_filters('rocket_buffer', $final);
}, 0);

add_filter('rocket_buffer', function($output) {
    //this is where changes should be made

    $pattern1 ='~(<img.*? alt=")("[^>]*>)~i'; // alt empty
    $alt_keyword = "BUBBLE";
    $replace_fkw = '$1'. $alt_keyword .'$2';

    return $content = preg_replace($pattern1, $replace_fkw, $output);
});

It supposed to replace img empty alt tag with given keyword which works fine except for the issue mentioned with cache plugins.

Any solution please.

Thanks