Why Polygon Collider 2D is becaming convex?

I have a sprite renderer with a .png texture.
I need to put a polygon collider surrounding the texture drawing (some black orthogonal lines ).

When the lines are orthogonal, the collider generated is convex, wich I do not want it to be.
When the lines are curves, the collider is correct.
See:



How to ensure the collider will surround exactly when I have straight lines?