Black outline around pre-comp when using a solid or shape as a background.

Also applying a simple choker makes the line disappears in after effects but rendering the video still results with the black line appear 

The black outlines only appear at certain percentages. and it's random, and sometimes part of or all of the line will disappear, but it still appears in the render itself. 

I don't think it's the track matte. here is the same comp with only  a random shape layer inside of it, and the outline in the comparison comp 

Start by viewing your artwork at 100%. This could be a fluke. That said, it probably has more to do with your track matte at the bottom than the background itself. I'm pretty certain it's some semi-transparent pixels from the matte only showing up under certain conditions and that can probably be fixed easily by treating it with Simple Choker or such.

Mylenium