There is already a comment on this in issue #91:
blapp commented on 12 Feb
I haven’t actually verified this, but a bit of browsing through the Android source code suggests this is caused by davdroid not currently assigning any groups to contacts (see #48).
where = VISIBLE_PHONEBOOK_WHERE;
VISIBLE_PHONEBOOK_WHERE = Phone.IN_VISIBLE_GROUP + “=1”;
“Flag that reflects the GROUP_VISIBLE state of any ContactsContract.CommonDataKinds.GroupMembership for this contact.”
The rationale for this is probably because with a Google/Gmail account, every email address that you’ve ever sent to/received from is added to your contact list in the “Other Contacts” group. To prevent all those from being synced to your car, that group’s VISIBLE flag would be set to false.
Best would be if Android changed the bluetooth sync logic to depend on whether a contact contains a phone number instead of whether it is a member of a group, but for now you might want to add group support to davdroid - even if it’s somewhat broken.