pfSense

If DDNS is enabled in a DHCP scope, a DHCP configuration stanza like this is created for the domain specified:

zone example.com. {
    primary 10.1.1.1;
    key "dhcp_update_key";
}

This appears to cause ALL scopes serving leases in that domain to subsequently cause DDNS updates even though they are configured on only one scope. This is likely due to the fact that ddns-updates defaults to on:

ddns-updates flag;

The ddns-updates parameter controls whether or not the server will attempt to do a DNS update
when a lease is confirmed. Set this to off if the server should not attempt to do updates
within a certain scope. The ddns-updates parameter is on by default. To disable DNS updates
in all scopes, it is preferable to use the ddns-update-style statement, setting the style to none.

I propose explicitly setting ddns-updates off|on for every scope based on the status of $dhcpifconf['ddnsupdate'] for that scope.