Hi, I have found this link https://meaninglive.com/2020/06/17/how-to-setup-dns-slave-auto-configuration-using-virtualmin-webmin/ The first part of this guide is in japanese, unfortnatly I don't know the japanese, but, later in the text, che core information are in english. The secret is the right configuration of the cluster master slave first, in the master, configure a new webmin -> webmin server index then configure the slave in servers -> bind dns server -> cluster slave server in this section...
I have a similar problem but, in my case, the zones from the master go to the slave out of any view, and therefore the service coundn't start / restart.