I have had an issue with the recent upgrade and Siri not responding in the car. This is a big issue as using CarPlay hands free without Siri responding renders it very ineffective. No one knew what the issue was. I have eventually found what the reason was on my phone:
To fix Apple CarPlay not responding to voice activation on iOS 27, open Settings → Face ID & Passcode → Allow Access When Locked, and ensure Siri is turned ON.
I hope this is useful to any others that may have been having the same issue. It would be a useful feature to be able to turn this ON from either the SIRI settings on the phone or in CarPlay. They are not currently.