Did you figure it out? This problem has pained me for couple of days now. I added empty SET nodes after remove from parent nodes in Transition to State macro after so when widget gets removed their variable gets invalid.
https://dl.dropboxusercontent.com/u/26177594/ShareX/2015/07/2015-07-26_04-03-22.png
But it didn’t fix the issue. 
This problem also appears in full Multiplayer Shootout example that’s downloadable from launcher. So whoever made it in Epic never tried to quit to menu from game?
Edit: I tried 4.7 version of this example and this bug doesn’t exist in that version. So it could be another 4.8 bug.