Thing is, that was a new jumper I just put on the spare monitor, so it
was quite unexpected that it was not making contact. But it was a case
of soldering a very tiny wire onto the back of a PC board where IC pins
stick out, so apparently getting a good weld was harder than usual. The
installed monitor with the old jumper had the same situation. 4 things
needed to mate: solder pad, IC pin, wire, solder, and I probably didn't
use the iron long enough in my concern about overheating the IC.
However, it was fortunate that it happened anew because I was
immediately able to know what was wrong with the old one!