I was going to post a question, but then figured out the answer by looking at the CalDAV log.
eM Client doesn’t support recurring tasks that have only a due date defined. For them to import properly, the start date must also be defined. I discovered this while syncing my Nextcloud CalDAV account, and noticed many of the tasks were missing. The log file showed lots of these errors:
DownloadItemsMultiget error Sync of item /caldav/path/to/task.ics failed due to the following error: Recurring component must have its Start defined.