Still happens in 0.9.0 build 1, and also, I noticed before that while mining blocks about 5 blocks away, they get destroyed much faster, and if you move in some direction while mining, blocks will continually destroy very fast.
It happens to me when I change voulme of my nexus 7 2013, but pressing the button on the right next to home btn and clicking minecraft again, the bar disapears again. It has happened in other apps as well, so I would call this android system bug, not mcpe bug.
Happened to me as well in creative on peaceful difficulty, but I think it's intended to work that way.
Hmm, I am pretty sure it was happening on other apps as well for me, when changing the volume, but can't confirm it now, I just updated my N7 2013 to 4.4.3 and installed xposed framework and immerse me module to get immersive mode in all apps and it doesn't happen anymore in any app for me. I did also, however, update to the mcpe 0.9.0 alpha build 3, so can't say for sure what fixed it.