The app is unable to guarantee the activation of immersive mode at all times upon startup, and there are more chances of failure than success.
Also, rendering is no longer correct once the app loses and regains focus (returning to home button, activating recent tasks) (refer to screenshot).
I have encountered similar problems before (when implementing this on my own fork), and was not able to rectify them correctly. I have not been able to perfectly replicate this bug as well on this build.
Will reply if any progress is achieved.
PS: This was encountered on Nexus 5 running on Android 5.0.2.