I see the same thing with my MLXe-4 routers. Rather than the modification @jh1kim suggested, I tested by changing line 50 from:
if (!$oids) {
to:
if (!$oids | $device['os'] == "ironware") {
and that causes it to work correctly.
This seems to be a bug in the device’s SNMP implementation, but it would be nice to correct for it in LibreNMS.