Comment 2 for bug 1711460

AFAIK, As arp-scan will show data for packets that went trough the interface, you won't see ARP request/reply to itself (as those will not "reach the wire"). Hence, the "missing" local IP/MAC pair.
IINM, You can observe same behavior on any other linux system and with other tools (netdiscover, tcpdump).
(I agree that this behavior does make additional cognitive load - It will easily show duplicate IP address for other hosts but for your own host you have to keep in mind that your IP/MAC is missing from the report)