We went for a different solution. As Ubuntu sends a DHCP release command just before a shuts down we noticed that even with the correct dhcp configuration the address still got released. We could prevent sending Ubuntu the release command by updating the /etc/systemd/networkd.conf file with the KeepConfiguration option. Downside is that as we have quit small subnet available for hosts we can’t handle really long lease times for DHCP, as the pool can get exhausted when recycle hosts.
For us this is working better (as we are using vSphere in this case):
https://www.virtualthoughts.co.uk/2020/03/29/rancher-vsphere-network-protocol-profiles-and-static-ip-addresses-for-k8s-nodes/ as this assigns always the same ip to a node as long as the node is not deleted.