Black line around stinger transition (track matte issue?)

Hi, I've been making a heap of stinger transitions for content creators and I've run into an issue.

I can't get rid of this small black line around where the stinger animation ends, and the new scene begins to show, as you can see here. 

From what I understand it's a transparency line where the 'cut out' starts/ends - but is there any way to close this gap? Blur or choking doesn't do the trick, nor does pre-composing the animations. I've made the entire thing in AE. 

Happy to share more info? Thanks!