Movie clip symbol does not scale smoothly

I made a walk cycle as a movie clip symbol. Using that loop, the character walks across/towards the screen and gets larger. But instead of a smooth scale (like in the tutorial), it only increases in size after each loop, creating a really bad jumpy effect. During the walk cycle the size doesn't change. Does anyone know how to work around this? The character's limbs are all movie clip symbols and I didn't use bones. Adding eases to the main timeline has not made any difference.

I'm using Animate CC 2017 with Windows 10 Home, made the animation following Adobe's "How to create a 2D character animation" tutorial.