I have the Bluetooth feature enabled for auto start/stop tracking, but the trips are not starting when my my iPhone connects to the Bluetooth device.

How to solve
Ensure that Travel Logs app is running before starting the vehicle as the app is not able to detect a bluetooth connection if the app has not started yet.

Also check you have Background App refresh enabled.

Background App Refresh
Device Settings -> General -> Background App Refresh (Please ensure this is enabled) -> Travel Logs (Ensure this is also enabled).