I know I’ve see it elsewhere, but this is the only reference to this behavior I can find at the moment:
"Thanks for letting me know that unplugging the iPhone has resolved the issue.
We have had a previous report describing similar behavior to what you’re seeing and we’re going to do some testing as to what could be causing this issue, I will be sure to update you on our findings."
From this thread.
I think Roon doesn’t (or didn’t) like it when you plug your iPhone into the Core machine.