I have been wondering about this issue myself so I contacted Uconnect customer service. I have the 8.4 Alpine system.
This was their reply:
I do apologize for any frustration involving your Uconnect system and being able to revert to different screens. It is always a good idea to reach out to us and I would be glad to help.
After some research here, your system lacks a specific way to operate as you describe, I am sorry. I have submitted your feedback here, we take customer comments very seriously, thank you for sharing that with us.
I dont think there is, I have to switch back after anything swaps the screen. It’s really been my only frustration with Carplay, and driving my mother’s Honda when I visit once in a while is enough to remind me how good it is all considered. Hahaha