Hello and welcome,
And that is why your launcher icon is not manjaro-specific.
Plasma 6 inherits its icons from the icon pack.
The ‘desktop theme’ doesnt matter like it did in Plasma 5.
In this case - you will get whatever Breeze sets as the “Application Launcher” icon.
So … choose a manjaro icon theme and you might get manjaro icons.
Otherwise … use the ‘configure’ functionality. Found in the right-click menu.
You can then browse to use any icon of your choosing.
(whether it be from some installed theme or from your Downloads folder)
It is not.
There have also been similar/related posts.
Such as here. Also found at this link are icons I use.