Black lines not in Preview showing up after Export

I'm creating a simple graphical animation that involves shape tweens to show circles forming with other circles.

In the preview, everything looks great. The three yellow circles move towards the blue circle in the center and change into their color, as seen below. They're using Shape Tweens with "Quad Ease-In-Out" as the effect.

However, after I export the .fla file to a .mp4 using Adobe Media Encoder, it looks like this:

For some reason, three black lines appear connecting the origin of the circles to their final destination. 

I did tons of searches, but I was unable to find someone who had a similar issue. Anybody know what is going on?