Why does the little down arrow beside the volume slidder disappears after a while?

[AC Question]

I recently updated to android 6.0 on my nexus 5.so far I encountered problems with the little down arrow beside the volume slider. after a while it disappears and the only way to get it back is to restart the device. other problems have to do with now on tap which doesn't work and instead I'm redirected to the google now page. I experienced some stuttering from time to time especially while the device is charging.