Where have my vector lines gone?

[Soquili]

That is a very interesting invisible line you have created Anas.

Following Egg's lead and converting the line to Shape, then breaking shapes both the line shape and the original filled shaped become invisible. Clicking on the Transparency Tool I saw that each of the shapes has a flat 100% Transparency applied. Removing the transparency from each shown two black filled shapes.

Using your original file and applying a bevel you see the 8pix gap between the shape and the bevel where the line around the shape should be. It is also interesting that I can select the background shape by clicking within that 8pix gap.