jpackage on Windows seemed to have problems with the non-standard Graal JDK file name. 
This would result in an extra directory being inserted something like...
runtime
	graalvm-ce-java11
			bin
			conf
			...
			
instead of...
runtime
	bin
	conf
	...
	
To workaround this I renamed the graal jvm to java-11. Despite the jdk-11 runtime being 
first in the PATH environment variable the jdk-16 one was default chosen to embed. 
So '--runtime-image /path/to/jdk-11' was required. The installation then ran 
without problems, no additional directory was inserted.