Hello zz9,
Thanks for reaching out in Apple Support Communities. We understand that your iPhone 8 thinks your car's Bluetooth is a headphone and the volume gets limited. We're happy to help. Have you received any alerts about your volume? If you have, this may be caused by headphone notifications. "These notifications give you more awareness around your listening habits to help you protect your hearing. When you receive a notification, you’ll hear a chime, and your volume will turn down automatically."
Check out the following support articles:
Adjust the volume on iPhone
Headphone notifications on your iPhone, iPod touch, or Apple Watch
In some countries you can't disable this warning. See the section, "Turn headphone notifications on or off."
Cheers!