I hit this same problem. The easiest workaround is to just upgrade everything to unstable (I did this and it fixed the problem). A better workaround is to upgrade all KDE packages to unstable (I cannot confirm this works, because I didn't do it). It's may just be the QML packages (e.g. qml-module-org-kde-runnermodel), which definitely need a tighter versioned dependence by plasma-workspace.