I did think of one more test, but it isn't one that I think will solve the problem, rather just a diagnosis. If you feel like taking the time, try creating an appointment in Outlook, exporting it to CSV, deleting the appointment in Outlook and then importing the CSV again.

What I predict (given the troubles we're having) is that the appointment you import from Outlook itself (that is, from the CSV that Outlook created) will have the same problem, ie the attendee fields won't be populated.

I haven't tried that myself yet, but I probably will. If my prediction is verified, I suppose it means that Outlook, whether by design or by bug, never did do that; the problem isn't in your method at all, but in Outlook itself.

If by some chance that import does work, then obviously we must carefully examine the two CSVs, one from Outlook and one from Excel, to see where they differ; because the CSV has no hidden codes, it's just plain text.