ExoPlayer stop/pause not working

try this...

if (exoPlayer != null) {
    exoPlayer.setPlayWhenReady(false);
    exoPlayer.stop();
    exoPlayer.seekTo(0);
}

I also went through same issue in my project. The issue I had was - Exoplayer is initialized twice when screen is loaded and one instance is released when we call realeseExoplayer() in onPause or onResume() as per SDK version. The second instance continues to play in the background even when activity is closed. So request to double check whether Exoplayer is initialized twice or not. If it is initialized more than once, make necessary changes to initialize it once.