SUMMARY
Adding the "shade" button to the right hand side of window titlebars causes the rightmost titlebar button (usually "close") to disappear when an application returns from fullscreen, as if pushed off the end of the titlebar. 
Titlebar is redrawn correctly if the window is later horizontally resized.
Observed with smplayer, VLC and gwenview, when using breeze or oxygen decorations, but not with plastik.
Removing the shade button from the titlebar or moving it to the left side restores expected behaviour.

STEPS TO REPRODUCE
1. Use the "breeze" or "oxygen" window decorations.
2. Add a "shade" button anywhere on the right hand side of the titlebar.
3. Launch one of the mentioned applications, enter fullscreen, then return to windowed.

OBSERVED RESULT
The rightmost titlebar button is either missing entirely or only partially visible, until the titlebar forced to redraw by resizing the window.

EXPECTED RESULT
Window titlebars are redrawn correctly as soon as an application returns from fullscreen.

SOFTWARE/OS VERSIONS
Platform: Gentoo GNU/Linux, openrc, X11. 
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.93.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION
Getting a window into this state then changing window decorations between breeze and oxygen does not redraw the titlebar correctly, instead the rightmost button reappears but the shade button itself vanishes. 
Horizontal resizing does not fix this condition, but  entering and exiting fullscreen again returns the titlebar to the original flavor of brokenness.

I'm not sure if all this is related to #450582, but it seems probable to me. IIRC it appeared around the same time too.