In After Effects, the mask outline is still visible in preview even though no layer is selected and

This looks like a result of being at a low Comp Resolution where, as such, you are seeing through to what's below your masked Layer.

In your screenshot., you are at a Custom Resolution which means something lower than rendering every 4 pixels horizontally and every 4 pixels vertically.

View your Compostion at Full Resoluion and you should no longer see the black line.

Here are some screen shots that illustrate what's happening.

Video by Andreas from Pixabay

Thanks for writing in, Jubilant_Zenith5FE3.

It doesn't look like the Mask path. Can you select all the layers, press M twice (it'll reveal the mask properties) and share a screenshot? You can try changing the Mask Expansion.

Let me know.