I changed the PDF-handling setting in Chrome from "Open PDFs in Chrome" (the setting that I've had all along) to "Download PDFs." With that setting, all attached PDFs in my checking register transactions give me the Save dialog, regardless of when they were attached. When I change it back to"Open PDFs in Chrome," it will open only attachments that were uploaded on 11/6 or before. Newer ones give me the Save dialog.

So it appears that the QBO program is ignoring my Chrome PDF-handling preference for any files attached after 11/6. But... as I think I've mentioned before, it exhibits exactly the same behavior with PNG files.