The VPN software is causing constant disconnects between my T-Mobile iPhone 6 (multiple iOS revisions) and my 2016 VW Golf. Disconnecting the VPN stops the constant disconnection cycle (i.e. you don't need to delete your VPN profile, just set it to not connect).
I discovered this after using multiple cables, having Apple replace the phone, and leaving the car with the VW dealer for several days (they updated the car's nav/entertainment software). After reverting all settings to default on the phone it became clear that activating the VPN would cause the phone to disconnect every minute or two (freeze, followed by connection lost/device not found, then connecting) and after constantly disconnecting for 30-45 min it would crash the car radio (black screen followed by startup sequence). Deactivating the VPN profile stops the problems. This happend connecting to different networks via VPN (office and Cloak).
So, deactivating the VPN will make the phone work with CarPlay, but it's not an acceptable solution. The Cloak VPN security app service team says they have seen this before and that it is some problem with Apple's iOS VPN implementation—but they have no idea what iOS could be doing to cause the problem.