When the same problem also happens in the Web browser and with Apple, it’s probably Nextcloud related. Did you already ask there?
Task/todo lists are correctly recognized in DavDroid. It’d be great if they could be exposed to the Android/Cyanogenmod calendar in order to allow displaying of deadlines in the calendar.
aCalDAV does this and it’s quite useful.
I’m using 1.3.1-ose on Cyanogenmod 13.
Thanks for your suggestion. This would be within the scope of the OpenTasks provider (and not DAVdroid), please follow up there.
FYI: aCalendar+ does this, using OpenTasks storage backend. (Not open source though.)