I did 2 things. The first didn't seem to have an effect, but maybe in combination with the 2nd, it did.

I used tips under "Mailmerge Data Format Problems"

First, I reordered the sheets, renaming them through VB Properties so that the sheet that has the data I want is Sheet1. This is based on what was said in Paragraph 4 of that section.

Second, in the very first record in the sheet, I made that field "not empty". Because that record is very old, it never gets selected for the mailmerge, but making the "middle name" field for that very first record "not empty" magically fixed the issue. I did also try making the first selected record have that field "not empty", and that worked as well. However, that is not something that makes my life easier.
This action was based on the information on Para 3.