Alpha over affected by alpha of render layer

I’m trying to pixelate part of my render with a box mask. Then I’m adding a white background color.
But the white is not added to the part where the unpixelated head is (see the black circle underneath the pixelated head).

Does anyone know what is causing this and how I can fix this, so the white background is also put underneath the pixelated area?

try to invert the box mask

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It only happens when I add the ‘Mix’ node. I have no clue why this is happening?

Without ‘Mix’ node works as expected:

But once I add the ‘Mix’ node I get the alpha of the original image:

Ok, I fixed is by rendering the body and the head on different scenes.