Audio Spectrum Quality

You have the softness set to 63% and the outside color set to black. Softness is giving you the soft edges. That should make sense to you. Using Polar Coordinates is further stretching the pixels the farther they get from the center so you get the trapezoid instead of a rectangle. You also set the Outside Color to black which cuts off half of the width and further softens the edges. If you change the softness to zero and set the Outside color to a yellow just slightly darker than the inside color you can put an edge on the graphics. Because you did not select the layer with effects and press the U key twice there is also a chance that the layer is scaled up which will also increase the softness of the edges.

That pale yellow against the white is not going to compress to an H.264 MP4 very well either. You need some better color choices. 

So there is no bug. You created the soft edges with the softness property, make the problem worse by using Polar Coordinates, made a poor color choice, and possibly scaled up the layer. 

If I wanted to use that yellow color and I wanted a sharp edge I would give it a darker outside color like this:

I rearranged the panels, showed the comp settings by selecting the comp in the Project Panel, pressed the U key twice with all layers selected to show all modified properties, made sure the Switches panel was visible and revealed the waveform of the audio layer by pressing LL to show you what the sound looks like. I have shown you everything you need to recreate that look except the specific color values for Inside and Outside Color.