WP Fastest Cache not reflecting changes made with add_filter() in functions.php

Hello!

I’m experiencing an issue with the WP Fastest Cache when using the add_filter() function in functions.php.

I utilize an MU Plugin named buffer.php that buffers the entire WordPress process and captures the final output for manipulation. Here’s the code for it:

<?php

/**
 * Output Buffering
 *
 * Buffers the entire WP process, capturing the final output for manipulation.
 */

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('final_output', $final);
}, 0);

In my functions.php, I’m using the buffer to manipulate HTML:

add_filter('final_output', 'filter_badge', 10);

The problem is, the cached version always shows the HTML content without executing the add_filter(). This happens regardless of whether I set the priority value to 8, 10 or 18.

Has anyone else encountered this problem or could offer some guidance? I appreciate any help.

Thanks in advance!