Even after a restart of kwin??

yes even after a reboot.....

Can you please attach your ~/.config/kwinrc ?

[Compositing]
OpenGLIsUnsafe=false

[Desktops]
Number=2
Rows=2

[Effect-CoverSwitch]
TabBox=false
TabBoxAlternative=false

[Effect-Cube]
BorderActivate=9
BorderActivateCylinder=9
BorderActivateSphere=9

[Effect-DesktopGrid]
BorderActivate=9

[Effect-FlipSwitch]
TabBox=true
TabBoxAlternative=false

[Effect-PresentWindows]
BorderActivate=9
BorderActivateAll=7
BorderActivateClass=9

[Effect-Zoom]
InitialZoom=1

[ElectricBorders]
Bottom=None
BottomLeft=ShowDesktop
BottomRight=None
Left=None
Right=None
Top=None
TopLeft=None
TopRight=None

[MouseBindings]
CommandActiveTitlebar1=Raise
CommandActiveTitlebar2=Start window tab drag
CommandActiveTitlebar3=Operations menu
CommandAll1=Move
CommandAll2=Toggle raise and lower
CommandAll3=Resize
CommandAllKey=Alt
CommandAllWheel=Nothing
CommandInactiveTitlebar1=Activate and raise
CommandInactiveTitlebar2=Start window tab drag
CommandInactiveTitlebar3=Operations menu
CommandTitlebarWheel=Change Opacity
CommandWindow1=Activate, raise and pass click
CommandWindow2=Activate and pass click
CommandWindow3=Activate and pass click
CommandWindowWheel=Scroll

[Plugins]
cubeEnabled=true
cubeslideEnabled=true
desktopchangeosdEnabled=false
desktopgridEnabled=false
flipswitchEnabled=true
glideEnabled=true
highlightwindowEnabled=true
kwin4_effect_fadedesktopEnabled=false
slideEnabled=false
windowgeometryEnabled=false

[Script-desktopchangeosd]
PopupHideDelay=1000
TextOnly=false

[TabBox]
ActivitiesMode=1
ApplicationsMode=0
BorderActivate=9
BorderAlternativeActivate=9
DesktopLayout=org.kde.breeze.desktop
DesktopListLayout=org.kde.breeze.desktop
DesktopMode=1
HighlightWindows=true
LayoutName=org.kde.breeze.desktop
MinimizedMode=0
MultiScreenMode=0
ShowDesktopMode=0
ShowTabBox=true
SwitchingMode=0

[TabBoxAlternative]
ActivitiesMode=1
ApplicationsMode=0
DesktopMode=1
HighlightWindows=true
LayoutName=org.kde.breeze.desktop
MinimizedMode=0
MultiScreenMode=0
ShowDesktopMode=0
ShowTabBox=true
SwitchingMode=0

[Windows]
ActiveMouseScreen=false
AutoRaise=false
AutoRaiseInterval=750
AutogroupInForeground=true
AutogroupSimilarWindows=false
BorderSnapZone=10
CenterSnapZone=0
ClickRaise=true
DelayFocusInterval=300
ElectricBorderCooldown=350
ElectricBorderCornerRatio=0.25
ElectricBorderDelay=150
ElectricBorderMaximize=true
ElectricBorderTiling=true
ElectricBorders=0
FocusPolicy=ClickToFocus
FocusStealingPreventionLevel=1
GeometryTip=false
HideUtilityWindowsForInactive=true
InactiveTabsSkipTaskbar=false
MaximizeButtonLeftClickCommand=Maximize
MaximizeButtonMiddleClickCommand=Maximize (vertical only)
MaximizeButtonRightClickCommand=Maximize (horizontal only)
NextFocusPrefersMouse=false
Placement=Smart
RollOverDesktops=false
SeparateScreenFocus=false
ShadeHover=false
ShadeHoverInterval=250
SnapOnlyWhenOverlapping=false
TitlebarDoubleClickCommand=Maximize
WindowSnapZone=10

Created attachment 105504 [details]
Screenshot

Since the bug was not updated for a while, here is a screenshot

What is the screenshot supposed to show?

The screenshot was taken while doing alt+tab. More precisely during the releasing of the keys. 

One can see that the "breeze" task switcher is there, whereas I use the flip switch;

This issue report is quite old. Can you please confirm, that it still persists with KDE 5.23?

I don't reproduce it. I think it is obsolete. Feel free to close the bug