pfSense

setdefaultgateway() may set IPv6 gateway as a IPv4 gateway in some cases,
see https://github.com/pfsense/pfsense/blob/master/src/etc/inc/gwlb.inc#L1209-L1220:

    if ($currentdefaultgwip != $gw['gateway']) {
        log_error("Default gateway setting {$gw['descr']} as default.");

        if ($ipprotocol == 'inet') {
            array_map('unlink', glob("{$g['tmp_path']}/*_defaultgw", GLOB_BRACE));
        } else {
            array_map('unlink', glob("{$g['tmp_path']}/*_defaultgwv6", GLOB_BRACE));
        }
        $defaultif = get_real_interface($gw['interface']);
        if ($defaultif) {
            @file_put_contents("{$g['tmp_path']}/{$defaultif}_defaultgw", $gw['gateway']);
        }

-- it always puts the new default gateway value (no matter of protocol family) to {$defaultif}_defaultgw