As far as I can remember, this never worked correctly in kwin 5:

In the best case, the "3D window galery" window switcher the window's reflections are never positioned correctly.

In the worst case, the galery appears to be completely messed up.

See the attached screenshots.

I'm running a multihead environment:

Screen 0: minimum 320 x 200, current 4160 x 1440, maximum 8192 x 8192
LVDS-1 connected 1600x900+2560+0 (normal left inverted right x axis y axis) 294mm x 165mm
   1600x900      59.94*+  50.00    40.00  
   1440x900      59.89  
   1360x768      59.80    59.96  
   1152x864      60.00  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   960x600       60.00  
   960x540       59.99  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   800x512       60.17  
   700x525       59.98  
   640x512       60.02  
   720x450       59.89  
   640x480       60.00    59.94  
   680x384       59.80    59.96  
   576x432       60.06  
   512x384       60.00  
   400x300       60.32    56.34  
   320x240       60.05  
VGA-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
   2048x1152     60.00  
   1920x1200     59.95  
   1920x1080     60.00    60.00    50.00    59.94    30.00    25.00    24.00    29.97    23.98  
   1920x1080i    60.00    50.00    59.94  
   1600x1200     60.00  
   1680x1050     59.88  
   1280x1024     75.02    60.02  
   1200x960      59.99  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x576i      50.00  
   720x480       60.00    59.94  
   720x480i      60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
   2560x1440     54.93* 
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)


ii  kwin                                                        4:5.10.2-0neon+16.04+xenial+build56                                              all          Transitional dummy for kwin-x11
ii  kwin-addons                                                 4:5.10.2-0neon+16.04+xenial+build59                                              amd64        additional desktop and window switchers for KWin
ii  kwin-common:amd64                                           4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager, common files
ii  kwin-data                                                   4:5.10.2-0neon+16.04+xenial+build56                                              all          KDE window manager data files
ii  kwin-style-breeze                                           4:5.10.2-0neon+16.04+xenial+build45                                              amd64        KWin Breeze Style
ii  kwin-x11                                                    4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager, X11 version
ii  libkwin4-effect-builtins1:amd64                             4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager effect builtins library
ii  libkwineffects11:amd64                                      4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager effects library
ii  libkwinglutils11:amd64                                      4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager gl utils library
ii  libkwinglutils6                                             4:5.4.3-0ubuntu1~ubuntu15.10~ppa1                                                amd64        KDE window manager gl utils library
ii  libkwinxrenderutils11:amd64                                 4:5.10.2-0neon+16.04+xenial+build56                                              amd64        KDE window manager render utils library
ii  libkwinxrenderutils6                                        4:5.4.3-0ubuntu1~ubuntu15.10~ppa1                                                amd64        KDE window manager render utils library
ii  qml-module-org-kde-kwindowsystem:amd64                      5.35.0-0neon+16.04+xenial+build35                                                amd64        provides integration of QML and KDE frameworks - kwindowsystem