Having slept on this problem, I have some additional thoughts.
First of all, RM suggested to me originally that a drag and drop of my entire database would fix the problem. And in fact it does. However, that is actually a very bad thing. What it means (and I have verified it this morning) is that individually customized citations are lost on a drag and drop and on a GEDCOM export/import. In other words, those customized Footnote, ShortFootnote, and Bibliography fields I talked about in RM’s CitationTable are lost on a drag and drop and on a GEDCOM export/import. So customized citations need to be added to the very long list of items that are lost on a drag and drop or on a GEDCOM export/import.
Second of all, I initially tested copying and pasting a customized citation using Paste With Reuse. Because of the way Paste With Reuse works, there would be no opportunity for the Paste With Reuse to fix the problem with customized citations not really being reset by the Customize => Reset to Defaults. However, with Paste With Copy, there would be an opportunity to fix the problem by not copying the customization fields. So I tested Paste With Copy this morning. Alas, using Paste With Copy leaves the citations customized. I suppose that’s the expected and intended behavior and I don’t think it’s a bug. The bug remains the failure by the Customize => Reset to Defaults process to clear the customization fields.