I've been having a recurring problem for months. I keep several address books within KAddressBook. Some of these are vCard directories and others are vCard files. The vCard directory address books are fine. However, I notice when I try to show the vCard file address books in KAddressBook that they're empty. I fire up Dolphin and go to the directory where I store the vCard file. Sure enough, the size is 0 bytes. The file's destroyed! Lucky for you guys, I keep regular backups of the files and can dig through my archives to retrieve them. As soon as I restore that file, I can see all of the contacts happily back in place when I open KAddressBook but, sure enough, the next time I go to check it the address book's been wiped again. Troubleshooting suggestions?
I am having exactly the same problem. It has persisted for about a year now, under both OpenSUSE LEAP 42.3 and LEAP 15.0. It is very annoying. At odd intervals -- I haven't yet figured out exactly when -- all the data I have saved in a VCARD resource gets wiped out. So I re-import the data from my backup file, and it's OK for a day or two, and then BOOM! all the data disappears once again. My current version of KAddressBook says 5.7.3, with libraries KDE Frameworks 5.45.0, Qt 5.9.4, and xcb windowing system, maintainer Laurent Montel.
5.7.3... we will release 5.12.0... I will investigate after my vacation.