If enabled DNS forwarder and the pptp users dialin from optional interface (not lan interface), well the DNS setting will corrected (see below red codes).
if (is_array($pptpdcfg['dnsserver']) && ($pptpdcfg['dnsserver'][0])) {
$mpdconf .= " set ipcp dns " . join(" ", $pptpdcfg['dnsserver']) . "\n";
} else if (isset($config['dnsmasq']['enable'])) {
$mpdconf .= " set ipcp dns " . $config['interfaces']['lan']['ipaddr'];
if ($syscfg['dnsserver'][0])
$mpdconf .= " " . $syscfg['dnsserver'][0];
$mpdconf .= "\n";
} else if (is_array($syscfg['dnsserver']) && ($syscfg['dnsserver'][0])) {
$mpdconf .= " set ipcp dns " . join(" ", array_slice($syscfg['dnsserver'], 0, 2)) . "\n";
}