Weird! Though I guess a lot of these would be sitting behind load balancers / reverse proxies anyway (so ipv4 is fine) and unlikely to up and change isps very often? Lol
If I had to, I’d be trying to add an extra cidr to one of the options listed at https://kubernetes.io/docs/concepts/services-networking/dual-stack/#configure-ipv4-ipv6-dual-stack But as you say, probably doesn’t work!
I use openwrt on my home network which uses dnsmasq for dhcp. It can give a static suffix which just works with the global prefix on the interface and the site local / ula prefix it uses