I am having this same issue. I updated to iOS 18.1.1 over the holiday and hopped in my car today. I drive a 2020 Audi Q5 and have an iPhone 14 pro. My vehicle is the model that was the last to design the multi-media user interface (MMI) as "touch-free" meaning there is a spinning wheel that is used to navigate the MMI screens, including CarPlay. When I connected to my car this morning, the CarPlay application worked as expected and I can navigate across the CarPlay app with the wheel. As soon as I tried to open a secondary app, like messages, phone, maps, Spodify, etc, the wheel selector stops working in the sense that I can spin the wheel, but not open anything, where as I previously could. I can still open apps on my phone while in the car and start a playlist or queue up directions. The data/information is still flowing through and interacting, but I've lost the ability to control car play using my car. This seems far more unsafe than turning a knob like a volume or air control. If my phone and car know I am driving, why lock one and not the other? And why lock the safer of the two approaches to control and leave the far more dangerous/distracting one unlocked? Make it make sense.