I have an iCloud calendar with several sub-calendars (private, business, hobby, etc.). I have mapped those to Outlook on web using the following instructions https://support.office.com/en-us/article/sync-your-icloud-calendar-with-outlook-for-mac-c9c67e41-274f-4527-ae5e-ea1003d89fc5?ui=en-US&rs=en-US&ad=US
Now the interesting thing is that I see all calendars in the Outlook apps on my iPad and iPhone, but only 2 out of 5 in Outlook on the web. Btw: I also did the same with a Gmail calendar, exactly the same behaviour. So I suppose it's rather an Outlook thing, but maybe someone in this forum has experienced the same…?