I’ve been having same problem since upgrading my project to 4.7. (Currently running 4.7.3) It also affects packaged projects running on two different computers (both running Windows 8.1 with all updates).
I was able to work around issue by adding an arbitrary delay of 1 second before calling Set Focus to Game Viewport node. I would still consider this to be a bug because slower computers might require a different delay so it leaves me feeling a bit uncomfortable to just pick 1 second out of thin air and because it wasn’t necessary before.