When i have to add a service to an open invoice the gst that was saved previously gets wiped out and have to go into all the services that where added and re apply them.

The reason your GST is being wiped out may be that the service item is set to non-taxable, or there's no default tax code assigned to that customer. To verify this, simply navigate to your Service item>Edit and check its tax status. Then, go to Customer Profile > Edit to ensure a default tax code has been assigned.