Microsoft notes, "This method works only when you use the Excel icon to open the application. If you use File Open within the
Excel application or double-click a file in Windows Explorer, the files will still open in the same instance as designed"
It's ridiculous and unacceptable to require users to employ a cumbersome workaround for an issue that has existed for over 3 years.
How strange. Yet our symptoms have disappeared, and we do open these files from Windows Explorer. Either the explanation is wrong or outdated, or there's something else going on.
Anyway I agree that it's ridiculous. However the user community seems to have accepted the level of support that is provided. It's a pattern we see with many contemporary software products, alas.