I can’t reproduce the problem with those steps:initialize a new address book on Synology CardDAV server 6.0.4-0080 add a contact in the Synology CardDAV Web Interface sync with DAVdroid change the contact on the Android device sync with DAVdroid again
In this case, everything works as expected. Maybe the problems are related to the imported contacts. Especially the X-ABUID fields look strange for me, maybe you should remove them before importing.
In any case, I can’t see a DAVdroid problem here.