Texture Paint Mode broken alpha problem

Recently, I have been trying Texture Paint Layer Manager add-on but I run into some strange problems. I have found that blender paints alpha in different way in UV editor and viewport. The problem is that the viewport painted images with alpha never look good.

Here is proof - I painted 3 strokes over white texture with alpha. UV painted strokes mix almost perfectly with blue cube but viewport painted strokes always have that white ‘feather’ effect. Also notice how different UV medium stroke looks with and without alpha compared to viewport stroke.

Is it bug or ‘feature’? How to deal with it?