"Registered from IP" contains invalid IP address

I recently found a registered user on my forum where the ACP claims the "registered from IP" is:

10.216.6.118, 161.148.54.145

The user_ip field in the database contains this exact string. When clicking the IP or Whois links in the ACP, PHP throws an error:

[phpBB Debug] PHP Notice: in file /includes/acp/acp_users.php on line 925: gethostbyaddr() [function.gethostbyaddr]: Address is not a valid IPv4 or IPv6 address

and

[phpBB Debug] PHP Notice: in file /includes/acp/acp_users.php on line 60: gethostbyaddr() [function.gethostbyaddr]: Address is not a valid IPv4 or IPv6 address

As another consequence, attempting to ban the user by IP fails with a green info block saying, "No IP addresses or hostnames defined".

I have "Validated X_FORWARDED_FOR header" enabled, and I think this might be related.

See attached screenshot for the IP appearing in ACP.

This bug has been placed in the ACP component, since it is the one most affected, but probably stems from Sessions.