Dark outline around transparent widget

Hi, have any one encountered similar strange dark outline around transparent widget?
the source images(png) has no black pixels, I simply gaussian blurred it




the material is the simplest

is there a way to remove the dark outline??
does it have anything to do with image format?
or forward shading?
similar things happens when I try to put it on to cutout human video, there is black outline around the skin, especially visible when in the VR goggle.

many thanks!

What does the texture look like with the alpha channel disabled?

You mean toggle this one?


it just became a full square.

That looks like it should work correctly. Not sure what the issue could be. I would use RenderDoc to capture a frame, and see if the issue is there when the UI render target is generated and how it gets rendered into the scene.