The db is here:
~/.local/share/system/privileged/Contacts/qtcontacts-sqlite
You need privileged access to see it, so
devel-su -p.
AFAICS there are three apps using it, jolla-contacts, jolla-messages, and voicecall-ui.
It’s likely one of the sync plugins (buteo) will also access it when that’s running.
Unfortunately I have no idea how to debug or investigate anything here, except maybe the sync process as described at