It seems that cfg.ControlPlaneConfig.ExtraSANs, in the case of control plane, is not set in the cloud-init userdata file /capi/etc/config.yaml.

See:
https://github.com/canonical/cluster-api-k8s/blob/main/pkg/ck8s/config_init.go#L28
https://github.com/canonical/cluster-api-k8s/blob/main/pkg/ck8s/config_init.go#L126

Example CK8SControlPlane that I used:

  spec:
    spec:
      controlPlane:
        extraSANs:
        - 172.18.0.2

And the resulted /capi/etc/config.yaml:

...
# Note here that the 192.168.100.2 is the control plane VIP
# There should be another entry 172.18.0.2, with the extraSANs defined above, but only the control plane vip is present
extra-sans:
- 192.168.100.2
...

There seems to be a correct implementation for join:
https://github.com/canonical/cluster-api-k8s/blob/main/pkg/ck8s/config_join.go#L18

Maybe I am missing something here?

Thanks.