I will ask myself:
The trick with this kind of “alphas” (the ones where background is 50% grey, like FlippedNormals SkinKit) is NOT TO USE THEM AS “ALPHAS”
Painter is different from Zbrush here.
The trick is to use them directly in the HEIGHT.Step by step for newbies like me:
Add a Layer (a Standard One, not a “Fill Layer”)
Inside this Layer add a Paint Effect
Select this Paint, go to Material Properties and load any of your SkinKit “alphas” (textures, really…)
Drop a single paint touch
Voilà! Here is your perfect bump-relief without that square border.
The brush alpha has nothing to do here. In fact it’s the standard “circular gradient shape”
You can drop your stamps one by one (scaling, rotating, etc) or play with Jitters. Total control and perfect results.
Alternatively you can also load a Filter inside you Height (like “Height Adjust”) and inside this filter add a new Input with your SkinKit desired texture. By this way, before stamping your bump, you can control the Offset and Multiply of your Height
Big thanks to @edenexposito for the tip. I was about to throw in the towel on this issue…