SOLVED!
The networking problem has been identified. Thank you, @_james_fuller!
This post replaces a previous post.
This post replaces a previous post.
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.
Mystery solved. I have no idea how long it would have taken me to get here by myself. Thank you, Jim! Thank you!