Allegedly, on or about 26 October 2016, Paul Smith sent:
Thanks, Tom, for your suggestion. By replacing my mouse with a different one, the reported problem disappears. Therefore, it is a hardware problem -- my first mouse is faulty.
It's a common problem. The buttons get a lot of clicking, and they're directly soldered onto the printed circuit board. So you have switch wear-out, as well as soldering fractures to contend with.