On every start Amarok's window is placed partially in both screens. It requires a next step of maximizing its window to exit from one screen and fill the other complete.
This is most likely a bug in your setup: how do you close Amarok? it has to be set to a specific screen, then closed with Ctrl+Q. The next time you launch it it should remember its setting, if not, this might be a system problem: which exact KDE or Plasma version do you use?
For a while it was the only application consistently opening in the middle of the screens. Just recently another application is showing similar symptom. That makes it more believable it is a KDE bug. I have filed this bug for KWin: Bug 385000 - On dual screen, application window goes in between
Thank you for the fast feedback, marking this as a duplicate of the KWin bug. *** This bug has been marked as a duplicate of bug 385000 ***