I have found other posts referring to the same problem. One, for example, goes back to more than four years ago:
Post title: Short Form appears for first reference instead of Long Form
Post date: 05-16-2013 09:28 PM
It seems to me that the developers have a couple of approaches available for dealing with this:
Suggestion #1: Use the “Title” field to distinguishrepeated citations in footnotes, in addition to the existing criteria (author, year). (Surprisingly, the “Title” field does not seem to be used to distinguish repeated citations in footnotes, although the help file claims otherwise.) This would seem to be the most straightforward solution.
Suggestion #2: Apply the settings from the “Duplicates” preference to the algorithm for detecting repeated citations in footnotes.