Custom build step won't be canceled

I'm using a "Custom Process Step" and after canceling the build process in QtCreator, it still keeps running in the background.

For simplification I used a GUI process e.g. "C:\Windows\System32\notepad.exe" and after canceling it says:
The process "C:\Windows\System32\notepad.exe" crashed.
Canceled build/deployment.

But the GUI (notepad) still remains open.