I would personally connect 2 Pc's into the MX device on those subnets and see if you can speak to each other over them. This would rule out your MX doing anything funky then work back and you will find your problem. As right now there is no solid genuine answer why its not working.  ... View more

That rule is spot on.    There should be no reason there is communication unless your switching has some funky layer 3 setup and is passing traffic between subnets before hitting the rule.   What is your switch setup? ... View more