the sync feature is meant for field groups that are automatically saved by ACF when the acf-json folder exists in the theme. There is a difference between the json files saved there and the json files that are created with the export. The files that are automatically saved contain a single field group while the export contains an array of field groups, even if there is only one group in the export. This is most likely why you can’t sync these groups or why it’s not working properly.
