Why are the DNS zones removed after transferring an account and removing the account from the source server?
In a DNS cluster relationship, changes to all zones will trigger actions on the linked servers depending on which relationships are configured.
For example, if the local server is a DNS server and a web server, removing the account removes the DNS from being active. To prevent this from happening, you should cluster DNS only with DNS Only servers. You should also configure web servers to use Write mode.
The below doc explains how to configure the DNS cluster using recommended methods.
Guide to DNS Cluster configurations
The DNS synchronize zones interface can be used to re-create a missing zone after removing the account from a related DNS cluster server.
You can also keep the DNS zone by selecting the option to keep the zone during account removal.
Terminate Accounts - Keep the DNS zones