I contacted Apple today regarding this issue and received some terrible customer service.
After being told to check the keyboard settings, I requested to speak with a senior advisor as the person I was speaking with hadn't even heard of this issue. This is how the conversation went after I asked to speak to a senior advisor:
(CS Rep)
Senior advisor says: This is a behavior that is not a ‘problem’, leave feedback at http://www.apple.com/feedback regarding this.
(Me)
So this is by design?
(CS Rep)
Not intentional if that is what you are asking.
(Me)
So it's not intentional... but not a problem? I'm confused.
Did Apple add this as a feature? Or is it a bug?
(CS Rep)
It is being reported but not as an issue/problem.
(Me)
But it is a problem...
And thousands of people agree.
(CS Rep)
Feedback to Apple will determine any next steps taken by engineers.
(Me)
So it's a bug. But you can't call it that?
(CS Rep)
Call it whatever you like and please make your feedback known.
(Me)
I am. And I'm talking to you trying to find a solution.
(CS Rep)
I apologize but I have no ‘solution’
This will be something Apple Engineers will ned to solve.
(Me)
I'll log a feedback report. But please don't play these "it's not a problem" games. Apple has broken long standing functionality. It is a problem. I'm now more frustrated than I was before I contacted support!
(CS Rep)
Way above my pay grade
(Me)
Okay, got it. Thank you for your time.
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Anyway, I've installed this app: http://milgra.com/high-sierra-media-key-enabler.html which solves the issue as long as I don't have both spotify and iTunes open at the same time.