The reason why is that you don’t have the post_id and ACF can’t get the post_id. You will need to somehow supply the post_id to the script that’s creating the CSS.
you can get the post ID in the head of a single post like this
$queried_object = get_queried_object();
$post_id = $queried_object->ID;
I don’t know you can get that to the php file, how are you calling it?
