Just to bring this episode to a (somewhat unsatisfactory) close: I tried all suggestions, to no avail.
Then suddenly, a couple of weeks or so ago, things started working normally, as you'd expect them to, without any problems of the kind that previously existed. I hesitated to report it in case it was just a temporary thing, but the now-correct operation seems to have remained in place.
Whether this is due to some automatic, bug-fixing, online update of Excel 2021, or Windows 11, or the mouse driver, or all of these, I can't say, but that would be my suspicion at this point.
So I thank you all for your interest in this case, and your advice and assistance. There's simply, now, no fault, any more.
Wishing you all a great New Year.