Do you see that same loss if you ping, for example, 8.8.8.8?
If not just edit the gateway and set the monitoring IP to that instead. Or some other external IP.
Do you have more than one WAN if not you can disable the monitoring action on the gateway. You will still get data but pfSense won't start reloading stuff if the loss goes over 20%.
It seems like maybe the Comcast gateway IP is just not good at responding to pings and it doesn't have to be. If they ping your modem from their end they won't necessarily see that loss.
Steve