I have a very simple implementation for this test.
LoginBackingBean has two methods which are called in 2 subsequent requests. Variable kept in flash memory during request 1 doesn't return in request 2 but in request 3.
To reproduce, "Simulate Login" button has to be clicked twice to retrieve from the flash. At first click, which is request 2, it returns null.
3 small files required to reproduce are attached. First referer.xhtml should be opened in a JSF app, then its login button calls doLogin method and takes you to Login.xhtml file. Now "Simulate Login" button need to be clicked twice to get the variable.