Weird line on video

For what it's worth, I don’t see any black lines either during preview or after publishing. If you're referring to a small black patch in the avatar's hair, it's likely that part of the avatar was unintentionally cut out during video production, possibly due to masking or keying, and ended up being rendered as transparent.

Since MP4 videos don’t support actual transparency, any transparent areas will appear black when imported into an authoring tool like Storyline. So what you're seeing is probably a byproduct of that — not an issue introduced by Storyline itself.

Can you point to the black line on your side? This is how I see it: