The Apple imposed restriction (by design, and under the pressure of distracted driving laws popping up) is that you can't give it a command that requires powering the screen on, looking at, or interacting with your phone (literally, it has to be 'Eyes Free'). The assumption they make is that opening any app would require turning the screen on and further interaction by the user, which is why that entire feature is "locked out".
That doesn't explain the post-pandora command bug you're seeing though but since it's a Siri response it looks like an Apple issue.