In 5.6 beta, the media player applet doesn't detect players launched after Plasma is started. The applet isn't shown in the systray and media controls are not available in the taskbar tooltip. Restarting plasma *after* the player is running makes the applet and the tooltip controls appear correctly. Reproducible: Always
Re-assigning to System Tray as that's the one responsible for DBus activation.
Oh, wait, task bar tooltips are also affected, so a bug in the mpris dataengine.
Sorry, turns out this is not related to Plasma 5.6 but to Qt 5.6 (I upgraded both at the same time). It's fixed for me by https://git.reviewboard.kde.org/r/127285/
Fixed by 70326bdbed9b3e2f74990e170cb400f1e6769c8d