Phone sync or swim?

I have an Android phone running Oxygen OS (OnePlus 5 brand/model). My sweetheart and I have identical 2016 Prius II cars, only the color being different. Strangely, when I am in her car and connected via bluetooth my phone syncs well with the car, including favorites, contact list, and call history. When I am in my car connected via bluetooth, I can make calls manually or via voice, and receive calls, but the favorites, contact list, and call history do not sync. If I try to sync manually under Settings, the sync starts and has an error at about 100 contacts. Is this a software version, or settings, or something else? Suggestions?