home button still leaves apps/games running in background??

I'm successfully playing Daydream content using my Samsung Galaxy S8.

When I'm done playing a game I don't see any other way to exit the game other than pressing Home. But when I remove the phone from the headset, I see all those games/apps that I had supposedly "closed" still running in the background.

How do I leave/close a game so that the OS process truly exits?