Using the ‘root’ server caldav.icloud.com will fetch calendar entries for a particular user, but the results will not always be complete. No errors are raised at all yet the results weren’t correct.
After a bit of experimentation it turns out using caldav.icloud.com instead of the user’s “actual” server (p02-caldav.icloud.com in my case) causes random calendar entries to not be fed back in the results.
Seems like something that should be fixed on Apple’s end. When you try to fetch the available calendars you get an explicit error if you use the wrong server, which makes total sense. But to return a 200 status but not return the full set of data?