If you're noticing parts of a prior scene appearing in the next during the preview or on the exported video, this may be due to the scene transition you have set. Please note that the Camera Panning transition adds the effect of making all scenes appear as though they're all drawn within the same big canvas, this is why prior scenes may peak through on the others. 

If you'd like to avoid this from happening, please go into Settings from the upper left of your timeline then change the scene transition to Swipe Mixed or whichever you prefer.