I just now realized that I never did provide the rest of the story; maybe, at the time, I was waiting to see if the problem really was gone.
I don’t know why I didn’t think of this and try it early on instead of wasting hours (and days) trying to get it sorted.
I occasionally create a system backup using Macrium Reflect.
I replaced my system with one of these images and have not had this problem since.
I may or may not have switched disks at that time; I cannot remember.