Ping reply goes out, never received (internet)

We have several locations with a basic cable modem internet connection where we use DMVPN. Our NOC monitors the broadband with a ping to the static IP. We set up routing to ensure the response goes back out the internet connection instead of the tunnel. On a very small number of locations (Like <5 of 650+) we see the ping arrive on the remote router and response go out, but the response never arrives back at the DC where the monitoring servers are. Remote locations are running Cradlepoint routers.

So far we've been assuming that some device on the internet in the path is dropping ICMP and there's nothing that we or the ISPs can do about it, but is there something we could look at to deal with this situation?