In my case, one of the main loops had a cable break under a roadway, in a place on the loop that was wholly Comcast infra rather than subscriber. You’re not wrong about the general case! But for that reason, they basically stopped acknowledging the issue to me at all, never followed up on support calls ever again, and it took them maybe three years to close that roadway overnight and fix their cable. (I was able to manifest the issue at a service speed of 125mbps when capacity up to 1+ gbps was available, but of course that low limit didn’t stop the modems from negotiating whatever full-width max-QAM links they could.)

Diagnostics mastery note: logically ruling out a readily testable possibility is only (somewhat) logical when one hasn’t exhausted all other possibilities. Displeasing and successful diagnostic tests that ought not to differentiate but do are how one exposes issues hiding in the blind spots of other experts. (If they hadn’t explicitly said ‘I have no ideas left’ in as many words, I probably wouldn’t have posted at all.) Here is an idea they hadn’t openly said they considered. The reasons this idea might or might not pan out are still interesting to me! TIL! But it was a beautiful and consumer-accessible scalpel of diagnostic and earned me a walkthrough of the signal contamination specifics by the senior truck tech who showed up to help the lesser truck tech, so perhaps it’ll help another.