CarPlay will lower radio volume.
CarPlay lowers radio volume, as if an Apple Maps or Waze voice prompt is about to happen. But, navigation voice prompts are off. Been happening for quite a while. And it’s annoying.
A simple restart of both your iPhone and car's infotainment system can sometimes clear up temporary glitches.
If that did not solve the issue then please get in touch with a reputed Car dealer and understand the reason behind it.