absolutely, i met same issue when i try to define some process vars by using java.lang.String or java.util.HashMap. after upload to guvnor, and import java.util.HashMap, in default, guvnor imports the java.lang.* package. success to compile. but if you click the button open and try to see the flow. you will get blank screen. i'm still investigating the root cause.
but, if you use String directly. it will display the flow you want.
in one word, please check those process vars again. or you can upload your files and i can check it with you.