UI element with transparent parts in the sprite bleeds into panel

Hello,

My simple setup is:
- Canvas
-- Panel (opaque dark grey)
--- Button (unity default button with uisprite with transparent sides, I just darkened the color) 

The problem is that the transparent parts of the Button sprite also makes the panel transparent.
This makes the panel look like it has a transparent border around the button.

See image.

See the light areas around the buttons, this is actually the background being visible through the panel.

I have tried various image import settings.
I have tried psd and png format with a square image and only defining transparency with the alpha channel.

It still has the transparency issues with the panel.

Has anyone any idea how to solve this?