Add trailingslashit() to $export_path

[jmslbam]

When leaving out the trailing slash the export becomes directorynamefilename.json instead of directory/filename.json and all files get exported to the same level.

Filter:

function wt_acfwpcli_fieldgroup_paths(){
    return [ get_template_directory_uri() . '/field-groups'];
}
add_filter('acfwpcli_fieldgroup_paths', 'wt_acfwpcli_fieldgroup_paths');