I do not sync field groups to all sites in a mutlisite installation. There is not really any need to do this.
This is what I do.
I create a theme with an acf-json folder and I activate this theme on only one site. This is my dev site. This is the only place where changes to a field group are made.
Any other site in the network that uses this theme is set up using a child theme. Part of the parent theme code is checking to see if a child theme is being loaded and if it is then it adds a load point to load the field groups from the parent theme. No syncing to these sites is needed.
