It only works with the Breeze window decorations, and it ignores — as in “doesn’t even save” — any other setting.
When checking the settings by right-clicking the blank applet, you get this… 
When changing the setting to how it’s supposed to be (and how it was before), the Apply button becomes unavailable.
Furthermore, closing the applet settings — whether you’ve changed anything or not — triggers a plasmashell restart.
Upon closer investigation, it appears to be a problem with the aurorae engine for drawing window decorations, or possibly with kdecoration itself.
For one, when not using Breeze, the buttons in the window decoration are now bigger than they used to be, even though they are set to “tiny”, and the “small” setting is no longer available either (except for Breeze).
This is how it used to be…
And this is what it looks like now…
Note: You must click on either image and then switch to the other one with the pager arrows in order to see the difference.
I’ve now decided to open a bug report at KDE about it.
i have the same error as you!
I switched to other custom layout’s the icon in left window tile returned.
As a work-around, under Colors and Themes>Window Decorations>Configure Titlebar Buttons
I removed the generic ugly button now. Better no icon at all as this generic ugly icon/button. But that’s my personal opinion.
I mean, i could also download another Window Decoration (there are other layout’s besides breeze working also, your assumption that this is a problem for non-Breeze themes are incorrect), but i still prefer my custom theme which i downloaded few year’s ago, which looks partial incompatible with the newer KDE version right now. 
At the end of the day, im not sure, if this problem in the playfield to fix from KDE or from the individual Window Theme Creator.
Most of the time, i look at this Global Themes and other Themes and im experience sadly the most layout’s going straight unmaintained from developer’s after a newer KDE Plasma version requires adaptations to stay up to date. 
I don’t use this widget and I don’t think I understand exactly what it does, but I think “Application Title Bar” might do the same/similar thing and I don’t think it has this issue (I haven’t noticed any at least).
When you maximize a window, then the buttons for closing, minimizing and unmaximizing appear in the top panel. Which buttons are shown can be selected by way of a slider, and of course, which style of buttons — i.e. from which of the installed window decorations.
It also works best if you set it to only show the window buttons in the panel if the maximized window is the currently active window, and in combination with the Truely [sic] Maximized kwin script, which removes the title bar from maximized windows.
Then I guessed correctly, and am using “Application Title Bar” for the same thing which hasn’t missed a beat. I have some vague memory that perhaps I did try this widget when I was setting up krohnkite and global menu, but went for “Application Title Bar” because it was better documented and seemed to perform better too.
