Black Lines Around Storyline Block In Rise

Hi all,

I've seen old posts about this with no clear solution.

When including a storyline block in rise, I want it to embed seamlessly into the background colour which it initially does.

When i then interact with the activity and press submit, black lines appear above and below the content. I've checked the size (16:9) and I've also checked in the feedback master and can't find any reason for this to be happening.

Any advice?

One image below without the black lines (before pressing submit), one with