I have 2 calendars - "Personal" and Google account's calendar. If I create a SubToDo in the Tree ToDo View and it belongs to the Google calendar (which happens by default), it is displayed as blank line in the tree. Switching to the flat view shows this SubToDo in the list. Swtching back to Tree view again simply does not show it at all.
I'm able to reproduce this issue as well via Kontact (v5.8.3) -> To-Do list. Note the sub-todo's work correctly if the todo is saved to the "Personal Calendar". Its only an issue if the event is part of the google calendar.
Same problem for me with DAV-Groupware-Ressource on Kontact/Akonadi 19.08.3 using DAV-Groupware-Ressources.