Why I see dark fringe/halo in scaled down RGBA imaged in Designer and not in any other soft?

If your image is straight then you should not have to use the premult to straight node.

Would you mind posting an image of the rgb and the alpha channel? Or even share a png that I can test on my side?

Thanks Nicolas, my 16bit PNG AOV renders including denoised ones are  indeed saved un-premultipied  by the render and I do use straight  blending in blend node.  When I use "premultiplied to straight"   which I think should be mostly useless on anything but 32 float image, I see strong white halos .    

And again , by default , if I drop the png exactly in its pixel scale  matching  substance output scale  it looks perfect , no halo or anything.    The problem occures when I need to scale png  or its original scale is not perfectly  x2.  So when I scale this png or any RGBA  I instantly see those blck halo appearing   while even in Painter I am not (mostly) and absolutly never in Photoshop  whatever scale i use with smart objects or just pixels .           

I assume  when RGBA is downscaled by 2d transform node or just by bitmap input node when its not x2  the black background from surrounding  pixel beyond un-premultiplied ones where alpha is perfectly zero still bleeds in.   Looks like all other image editors somehow deal with that. Painter included. 

There is technically nothing wrong in having the black halo, it just indicates your image is premult but the 2d view is not rendering it as a premult, there is a little button at the bottom of the 2d view to display the alpha as premult.

If you prefer working with straight alpha, you can use the premult to straight node before applying the downscale.