If your main site and sub sites all use the same theme and the only place that you edit the field groups is on the main site then there is no reason at all that they should be synced and in fact should not exist in the DB for the sub sites at all because the DB version of the group will never be used. The only time the DB version of a field group is used is when you are editing that group in the ACF admin.
That being said you should look at some of these results. This topic has been covered many times – search results
