The ONLY view
that worked for my case of having multiple activities was:
activity.window.decorView.findViewById(android.R.id.content)
I had the same problem with the achievement popup. The "Welcome Back" popup was showing correctly by just using my own View, but once I started opening other screens where I wanted to show the achievement unlocked popup, it was not working. The only thing that ended up working was using the content
view from the decorView
val gamesClient = Games.getGamesClient(activity, googleSignInAccount)
gamesClient.setGravityForPopups(Gravity.TOP or Gravity.CENTER_HORIZONTAL)
gamesClient.setViewForPopups(activity.window.decorView.findViewById(android.R.id.content))
You can call this code from any new activity you open and the pop-up will show up there as soon as you unlock your achievements.