Sub tasks in Apple Reminders

The integration of Nextcloud’s Caldav implementation in Apple Reminders on MacOS works, but there is no hierarchy with sub tasks. Sub tasks from Nextcloud show as normal tasks in Apple Reminders:

Apple Reminders seems to (falsely?) detect that sub tasks are not supported by showing a greyed out “Indent Reminder” and “Outdent Reminder”:

My question is if this is due to the lacking RFC 4791 implementation of Caldav in Nextcloud or if it is some quirky VTODO Apple Reminders issue.

The Calendar and Tasks apps in Nextcloud are just CalDAV clients, so I would say limited compatibility of the new Reminders apps on Apple (which are more designed to work with their iCloud proprietary APIs these days).