You could try a network reset. That will restore the bluetooth adapter in your phone to factory, unfortunately that also happens with wifi as well and you would need to reconnect to all wifi networks and bluetooth devices.
Android auto still relies on the bluetooth connection to handle the calls, even if you're plugged in.
Being the issue is primarily calling I'm wondering if some sort of incompatibility with the hands free profile that either the phone, or sync is using. Wideband audio (HD Voice) uses HFP 1.6. Media relies on A2DP and wouldn't necessarily be affected by an issue with the other. Were any phone updates applied prior to this? What build number is the S8+ on? You'll find that in that in about phone, and under the software section. If you can get the build number I can reach out to some old support team contacts when I used to rep for samsung and did quite a bit of troubleshooting and see if they have any experience with the issue and the S8+. Unfortunately I left the company just a couple months after the S8 launch and didn't do much aside from sales support with those.
Does your wife have the same model phone? If not then below likely isn't of much use as the odds of 2 random phones w the exact same issues are fairly low.
If she does you could try to kill it with fire so to speak. If you have a SD card or a sufficiently large USB drive and your OTG adapter that came on the box with the phone you can use the Smart Switch application that's hiding in the settings menu to essentially clone the phone to external storage and then do a factory reset. Once your back up and running and before signing in/installing anything pair back up to the vehicle and see if the issue persists, if not, then restore from the backup and if it is still gone then you know it was software/phone related though you'll likely not discover the root cause. If not at least you know what it isn't and can continue to eliminate variables.
The smart switch application restore will put all settings, wifi networks, apps, photos, videos, and home screen layouts back where they should be. Bluetooth will need paired again and you'll need to enter account passwords for things you use should you go that route. Be sure to have those handy before doing anything like that and as always backup any sensitive data to another location just in case.
