The attached video should make clear what I mean when I say that the dimmed window appears during the animation for switching a virtual desktop. The bug does not arise when the effect is disabled.
Created attachment 127454 [details] video of issue
It might be relevant that my AnimationDurationFactor is set to 1.6 in kdeglobals.
This bug was reported against an outdated version of KWin. We have made many changes since the. If the issue persists in newer versions can you reopen the bug report updating the version number.