While exploring the issue I submitted above, another problem arose: I defined the sidebar to appear “For Archives” and under “Archive Types” selected the “Category Archives” type. When I tried to remove this definition by unclicking the “As sidebar for selected archive types” and <Save>, the definition remains. I can add other archive types and delete them, but I can’t delete the “Category Archives” type nor remove the sidebar from pages with Archive Type=Category.
I tried to create a new test sidebar, and set it to appear “For Archives” and under “Archive Types” with Category Archives chosen. I can’t delete this definition from the new test sidebar, but this does remove that “sticky” definition from my working sidebar.
I’ll just delete the test sidebar to clean up my site, but this looks like a bug.