Validation fails in server's DHCP zones tab

After adding two MAC:IP mappings in a DHCP configuration for the same system, they appear correctly in the system's DHCP tab.

When validating a modification in this tab, an error is displayed: The value "xx:xx:xx:xx:xx:xx" for field "" is not in the list of possible choices. The second MAC address is not recognized (the one not in the systems/server tab, but only in the DHCP configuration) and is not listed in the drop-down menu below.