If you are running with a separate modem (or a router in bridge mode) and router, which device is responding to the pings?
I had always assumed the modem. However the ZyXel VMG1312-B10A I am using in bridge mode does not have ICMP enabled for WAN traffic, which I seem to recall was a necessity to get the necessary BQM ping response on my VMG 8924-B10A, when used as a combined modem/router.
My BQM responses have also lost the overnight long grass effect overnight, once I swapped the 8924 (router only) for a Netgear Orbi router.
I suspect I have answered my own question because the above evidence suggests that it has to be the router responding, which somewhat bears low my original assumption!