SOLVED!

This morning, Jim Fuller took an interest in my networking problem. We happened to be catching up on some work stuff and I described the problem. He agreed it was baffling.

As we talked through the possibities, Jim remarked that cables sometimes have issues. I don’t remember exactly how it came up, but the idea that the cable was acting as some kind of weird antenna when it was unplugged surfaced.

Easy enough to test, I unplugged the cable from the router.

The problem did not occur.

I plugged it back into the router and unplugged the laptop end,

The problem did occur.

I have a one of those USB hub things because my laptop only has USB-C connectors. If this was a mystery novel, this would be the point at which a suspect came into sharp focus as the prime suspect!

I unplugged the cable from the hub.

The problem went away.

The culprit

Mystery solved. I have no idea how long it would have taken me to get here by myself. Thank you, Jim! Thank you!