Hey, just tried to install it myself and did it like this, which seems to work:
function ACFProSetDir($dir) {
$dir = get_template_directory_uri() . '/plugins/acf/';
return $dir;
}
add_filter('acf/settings/dir', 'ACFProSetDir');
In short: I only used acf/settings/dir and instead of setting a dir, I’ve set a path? Kinda weird, but seems to work.