I have a Raspberry pi 4 with Manjaro KDE, and it runs very well.
The problem is that sometimes when it shutting down the process stops for several minutes on the wpa_supplicant and network_manager line.
This is not always, maybe half of the times.
I do not use KDE but have not noticed that here. It is weird that it would do that 50% of the time and be ok the rest of the time. Do you have crda installed and your Country code configured?
I am running Manjaro 5.15 KDE now. This slowdown shutdown appears to have been managed. Prior to this, my kernels were doing the exact same thing. I would need to press the CPU reset button to get out of the endless wpa_supplicant and NetworkManager shutdown loop.
This leads me to believe it is a kernel bug, and nothing my own local machine is doing.