Did you manage to find the root cause of the issue ? 

Did you meant the return traffic is getting dropped coming from Router towards switch ?

I understand that you are not able to reach internet from the downstream switch that is directly connected to the router.   I guess the ping traffic that you initiate is passing through  sub-interface always irrespective of where you apply the policy...

Please share the configuration templates that you are using with sub-interfaces and physical interfaces ?   I assume, you have received default route from ISP - Gi-0/0 interface. Share me the source IP and destination IP address  for the ping that yo...