This was working correctly in Blender Internal. There is an important difference between the way "Selected to Active" bakes work in Blender Internal vs. Cycles: Blender Internal would render the active object into the bake map, whereas Cycles does not. That might be the source of the problem described here; it's possible that BI and Cycles are using the same approach to render the margin around the baked result, but because Cycles doesn't render the active object in Selected to Active mode, that baked result doesn't match up with the UV islands.

I really hope that makes sense. Maybe some images will help:

Blender Internal, Selected to Active, Margin 0:

Blender Internal, Selected to Active, Margin 64:

Cycles, Selected to Active, Margin 0:

Cycles, Selected to Active, Margin 64:

It would be great if we could get a checkbox to enable or disable rendering of the active object into the baked map, but I guess that's more a feature request than a bug report. Anyway, what we need is for the margin to render around UV islands, not around baked details that don't reach the edges of the UV islands.

poke

I can confirm in 2.82.
Basically, in the image shown, there should be no red (margin from the previous bake) in the areas where you have uv face.

poke
any news with it?