It is not a bug - your issue is typical for KDE - they do everything a lot of things different.
As far as I know there is several deprecated locations for the file.
XDG_CONFIG_HOME is $HOME/.config and as such the correct location.
So if this is creating issues for KDE then it is a KDE problem - not an xdg problem.
Please note the following quote from the above link
Applications are free to ignore or only partially implement the XDG standard.
Which may exactly be the issue here - KDE are implementing their own interpretation which is overridden by the XDG specification which is implemented by Arch based systems.