See the referenced Debian bug. I had the same problem with sys-apps/net-tools-1.60_p20071202044231-r1 and applied the suggested patch, which fixed the problem. Apparently this hasn't been addressed in Gentoo. On a system which periodically renews ipv6 interfaces, ifconfig quit showing inet6 address stats for interfaces, as described in this bug, but after applying this fix the problem disappeared. Perhaps this patch should also be applied to net-tools in Gentoo.
added patch from Debian to 1.60_p20090728014017