The Gateway dashboard element shows the percentage of loss based on ICMP echo requests to your default gateway (by default). You could try changing the IP address that you use for monitoring, under System -> Routing on the Gateways tab, you would edit your gateway to change this setting.
If I were in your shoes, I'd ping the ISP's default gateway and then show them the statistics from ping. You can do this in pfSense under Diagnostics -> Ping. Assuming that you allow ICMP echo requests on your WAN port, they should be able to ping you too and see the loss for themselves.