When compiling net-dns/dnsmasq-2.80[static,!dnssec] sys-libs/gmp[static-libs] is needed although the message "* Only sys-libs/gmp and dev-libs/nettle are statically linked. * Thus, dnsmasq-2.80[!dnssec,static] makes no sense; the static USE flag is ignored. " suggests otherwise.
With USE=dnssec, dev-libs/nettle becomes a dependency. With the additional USE=static, dnsmasq will statically link to libnettle and consequently libgmp during the build. Without USE=dnssec, dnsmasq will not link statically to anything (not even libc?) so USE=static is ignored when USE=-dnssec.