The dhcpv6 client can be running to manage the interface address, but it can also be running to manage "parameters-only" and/or prefix-delegations. It can be useful to restart the client even if it's not managing the interface address, but the current code gates on the presence of `address dhcpv6` in the config:
```
set interfaces ethernet eth0 address 'dhcp'
set interfaces ethernet eth0 dhcpv6-options parameters-only
set interfaces ethernet eth0 dhcpv6-options pd 0 interface dum4 address '1'
set interfaces ethernet eth0 dhcpv6-options pd 0 interface dum4 sla-id '5'
set interfaces ethernet eth0 dhcpv6-options pd 0 length '56'
set interfaces ethernet eth0 dhcpv6-options rapid-commit
set interfaces ethernet eth0 ipv6 address autoconf
$ renew dhcpv6 interface eth0
DHCPv6 client not configured on interface eth0!
```