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Was the iCloud calendar created in Outlook desktop application after installing the software Apple provided?
If the calendar group and the calendar are created manually at Outlook desktop application, the calendar under the new calendar group can work at OWA properly. Based on my experience, if the calendar and the calendar group are created via third-party tools, it may cause some issues. In this case, there is no official document about integration with iCloud calendar in OWA currently.
However, we also welcome other community members to involve this case to provide useful information and suggestion.