No, you do not need to sync the files.
You edit a field group on the main site. The field group will never appear on the sub site to be edited and you would never edit them here. The existence of the group in the acf-json folder is all that is required, they will be loaded and used from there. If you are never going to edit the field groups on one of the sub sites then adding them to the database would be a waste of space. Using the same theme they will be already “synced”
