Syncing is to sync the database with the json file, not the other way round. If a JSON version of the field group exists this will always be used everywhere except when editing the field group in the admin.
The only purpose of the syncing is to update the field group so you can edit it without destroying changes in the existing JSON file.
The field group does not even need to exist in the database on the staging site. You could delete all of the field groups from the staging site through ACF and then upload all of the json files from the local dev site. The db version on the staging site is unnecessary overhead if you don’t plan on editing them there.
