Yes, I tried those steps. It did not help.

It turns out the problem was that my bookkeeper had created "credit memos," and the numbers assigned to those credit memos were also treated as *invoice numbers* within QBO. However, searching for those numbers, including using various versions of advanced search, would not show them. They didn't show up under invoices or transactions. I had to know that I needed to look for a credit memo, before performing a search uncovered them.

So this was a confusing waste of a couple hours of my time, since QBO decided a credit memo and an invoice are the same but credit memos are somewhat hidden.