After editing two contacts in Nextcloud 13.0.2 hosted on a Debian derivative, I tried to sync back to my Android device running DAVdroid 1.11-ose downloaded from F-Droid. The result was that all contacts on the device (Samsung Note 3 running Android 5.x) were removed, even from all other accounts (Device, Google, Telegram, SIM, and Nextcloud). The data remains on the Nextcloud server.
Here is the debug log:
I’m happy to help to reproduce the bug and triage solutions as need be.