Video changes aspect ratios during playback

Make sure that the all videos in the AVS have the same aspect ratio.

Make sure that the Adaptive Video Set you are using only contains videos with the same aspect ratio. Select one of the Adaptive Video transcoding presets available in the job options at upload time. If your existing AVS contains videos with multiple aspect ratios, edit it to remove the ones you don't need. When you are done, save and publish to push out the updated set.