Hey guys,
So after testing I believe I have found the reason as to why this is occurring. Since your border/texture you are trying to render has translucency, you need to set the Blend Mode within the Actor Blueprint for the Widget itself from ‘Masked’ to ‘Transparent’, as seen in the below image.
Blend Mode - Transparent
Border Alpha 0.3
As you can see it is rendering as expected. Since you are using a gradient from 1.0 to 0.0 for opacity, using the masked blend mode is going to clip out anything up to the masked clipping value/threshold. This is why you need to use the translucent blend mode so you can render the entire gradient.
Let me know if you have further questions.
Thank you,