CarPlay switched to phone audio input after Siri voice control regardless of what source was playing before engaging Siri
When using “Hey Siri” to text or open the garage door or whatever, CarPlay returns to the phone as the audio source - regardless of what was playing when you activated Siri. Radio, Sirius XM, CD player, no matter, it goes to phone input after the command is processed.
iPhone 12, iOS 17