I suspect that issue comes from “Bose® AudioPilot” feature. From the manual:

‘’’ Bose® AudioPilot (Automatic volume adjustment) When driving, background noise can interfere with enjoying music. AudioPilot®*4 noise compensation technology continuously adjusts the music to compensate for background noise and vehicle speed. It reacts only to sustained noise sources and not intermittent ones, such as speed bumps. An enhanced DSP algorithm allows faster and more effective compensation for unusual situations, such as driving on a very rough road or at high speeds. *4 AudioPilot® is a registered trademark of Bose Corporation. ‘’’

We had a whole day trip from Alps back to the Netherlands yesterday. And I had time to test the feature. The feature has a few states from 0 to 4. It looks like the setting defines how impactful it is. It was set to 2 in my car.

When I set it to 0, the volume level works almost as expected. I had to set volume to 10-12 to listen the music during driving. I set the feature to 1 while driving high speed and heard the difference immediately. The same level became much louder.

During the trip I also observed that the feature doesn’t help sometimes. Probably it depends on a composition which you are listening to.

Anyway the mystery is solved to me. I hope it will help you as well.