When you do right mouse click on some window in taskbar -> click on "Move To Desktop" -> click on "Move To Current Desktop" = result is All Desktop. If you repeat it again than it moves only to Current Desktop. So the Move To Current Desktop is not working on first pick, it moves to All Desktop instead. The action has to be repeated to get it work. This is present from 5.8.2(or even before) and is still present in 5.8.4. Confirmed from more users before opening the bug here.
Git commit 039ab6aad9b0c0e51e84bc5104cdfb00d93b3c97 by Eike Hein. Committed on 26/11/2016 at 18:49. Pushed by hein into branch 'Plasma/5.8'. Fix "Move to Current Desktop" action moving to all desktops instead. M +1 -1 applets/taskmanager/package/contents/ui/ContextMenu.qml https://commits.kde.org/plasma-desktop/039ab6aad9b0c0e51e84bc5104cdfb00d93b3c97
Confirmed, solves the issue. Changed it in: /usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/ContextMenu.qml until the new package arrives. Tnx!