SUMMARY
Window placemnet is set to "Minimal overlapping".
I have two screens, 1920x1080 and 2560x1440 -- more than enough space, one should think.
Frequently I keep a small-ish (about 1000x800) window pinned in one corner with "Keep above others" enabled.
if another application opens a dialog, or if I open an app from systray, those windows frequently appear _behind_ the above mentioned window!
This would be incomprehensible in every case, since 
a) the window behind which it appears is set to "Keep above others", thus it clearly will be hiding the new window
b) there's more than enough space elswehere on those two screens

Additionally, a window placement called "Minimal overlapping" in itself should ensure that windows do not appear 
a) completely behind or on top of each other, but always show some part to make them available in any case
b) not overlapping at all 
as long as there's free space on the available screen estate -- and usually there's plenty.

In any case _never_ behind a window set to "Keep above others" -- that certainly is plain pointless. 
That the whole desktop should be covered by "Keep above others" windows is hardly plausible, thus any window with that property is out of bounds.

Operating System: Debian GNU/Linux
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
Kernel Version: 5.10.0-4-amd64
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4700MQ CPU @ 2.40GHz
Memory: 15,4 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4600