Nofollow external links

Edit 2: By suggest of @Mark Kaplun, the general solution should be this. We will do filtering after the page is generated, so we won’t care about which plugin we’re using.

We need a hack here to get whole page:

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);

I take this code from this question

Then you can remove 2 old filter:

add_filter('the_content', 'my_nofollow');
add_filter('the_excerpt', 'my_nofollow');

Use only one instead:

add_filter('final_output', 'my_nofollow');

Old answer

Just add one more filter.

add_filter('acf/load_value/name=my_field', 'my_nofollow' );

Edit: The concept to solve the problem here is filter the content that generated by plugin, for you here is ACF. Because the_content is from WP, not the plugin you’re using, so add filter to the_content won’t help.