Shapes disappear off screen on iPhone 5S

Im not sure if its supposed to do this. While my 10 month old is playing with it the shapes will originally have 3 on the screen and then 1 just disappears off the screen when theyre bouncing around. Eventually there are no shapes visible on the screen, but the app still vibrates as though shapes are still bouncing and hitting the sides. I have to close the app and open it again to get them back. Other than that I love this app.

Angryfoodguy about Smart Tot Rattle - Cause And Effect Sensory Stimulation