The docs for /etc/config/dhcp says that the "mac" option is: The hardware address(es) of this host, separated by commas.
In practice, when using commas the GUI will highlight in red the offending hosts.
This is either a minor cosmetic issue in the GUI or a minor mistake in the documentation, since the IPV4 static lease is correctly assigned to either wifi or ethernet interface in the laptop, regardless of the separator used for multiple MACs.