That sounds like a bug, probably in the Kia software, but could be an interaction problem. If I'm reading your post correctly, it appears you have already found a workaround. Another way to fix it is to simply plug your phone into USB.
Android or iPhone? I have some issues with what happens when I stop the car, or start the car with a previously playing streaming program. Sometimes the radio comes on, sometimes the handset starts playing, and other random events that are not the way I would have done the programming. But I've never seen your particular issue using Bluetooth or plugged into USB with my iPhone. Carplay has its own issues. For example, if I switch to radio, Carplay overrides that action and a second later resumes previous stream.
2018 Kia LX HEV Metal Stream with Advanced Tech