ipv6 is not mandatory on lxd. I was suggest that if I don't need ipv6, I just need do add an ipv6 address on the bridge. https://github.com/lxc/lxd/issues/3688 So please add an ipv6 USE that, if enabled, wants the depend/rdepend with ipv6, otherwise no.
Fixed in https://github.com/gentoo/gentoo/pull/5621