I've diagnosed it further. When Pandora is launched automatically by the Bluetooth connection with my car stereo, It does not open an app card for Pandora, so there is no way to close Pandora when I exit my car by swiping away the open app card. Closing the playback controls does not close the app. Nor does turning off Bluetooth (which really should be the trigger). Pandora really needs to fix this.
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