Thanks for the explanation.
How is the Group Management/Clients handled internally? If I set up groups/domains/adlists for a specific client I do it via web GUI displaying IPs. But with the possibility of changing IPs (by DHCP) you would need to store (and use) the MAC internally instead of the IP to identify a client and apply the right rules. Otherwise you would not be sure that the same physical device rules were created today will be applied tomorrow...
Suggestion to the issue, assuming someone with a "not-simple" network setup should know how to set static IPs or fixed DHCP leases:
For clients on the same subnet don't change anything. But add clients making requests to pihole in the network overview (even though you don't have the MAC - write "N/A different VLAN/subnet") and add the ability to delete them (similar as someone requested here).
In the Group Management/Clients show them in the drop down list as well, but separate them from the ones from the neigh cache (e.g. horizontal line) and add a statement that static ip/fixed DHCP should be used for these devices.