Hello,
I’m using DAVdroid 1.6.5-play on my Samsung Galaxy A3 (2017) with Android 6.0.1 to synchronize Calendar, Contacts and Tasks with nextcloud 12.0.1.
Every time I close the OpenTask app manually, all tasks are gone. Then DAVdroid starts to synchronize for minutes while the tasks slowly appear again one after the other.
I already reinstalled the OpenTasks app and DAVdroid.