Hi GraemeW,
Thanks for your detailed description. I've done some tests on my side. The below are the calendars in my iCloud. Home calendar and Work calendar are the default calendars in my iCloud. I also created the calendar "TestCal" for test.
Based on my tests, only the events of the calendar "Home" can't show on Office 365. The events of the calendars "Work" and "TestCal" can show on Office 365. I've tested on several Office 365 accounts and have the same results.
Given this issue, may I confirm whether you have the same results as mine? Could you create a new calendar for test and let me know if it works on Office 365?
Best Regards,
Aaron Yuan