I too have been trolling various forums/websites since I upgraded my iPhone 13 to iOS 18.1, and yes, to find a solution to this same change in CarPlay integration behavior. Prior to upgrading my iPhone 13 running iOS 17.7.1 to iOS 18.1, CarPlay worked just fine.
Immediately after I installed the new iOS 18.1 update, like a light switch it changed to this new behavior.
I too have tried everything including:
- Enabling/disabling the RCS protocol (as this was mentioned as possible cause), no change
- Went through all possible notification options/combinations (on both my iPhone 13 and within CarPlay), no change.
However, I do get an audible tone indicating a text message was received, but (if not on the CarPlay screen) no longer receive a popup message.
The most probable answer (which is mentioned on several sites), is this new behavior is by design.
The "why" is still unclear; possibly to force the user to use Siri message announcements, or to influence more CarPlay screen time in your vehicle. It's another example of how a behavioral change in software can influence an unconscious change in human behavior.
Just a thought.