See the problem below, Blue layer is clipped with the grey background but as soon the blue shape layer starts "Shadow Intensity is low"
Here is a closeup of the problem
Okay here is the Layer Panel for this
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That's an optical illusion - if you zoom in really close, you'll notice that the shadow is actually the same, but because of the higher contrast between the white part and the shadow, it seems to have a contour that the blue part does not (at least that's what it looks like for me).
What you can do (it's a bit of a workaround, I know) is:
Apply a Stroke to the white part of the rectangle (I'm assuming that the blue layer is the Clipping Mask, not the other way around).
Group only the white layer.
Apply the Drop Shadow to the group.
Make the blue layer a Clipping Mask (it needs to be directly above the group). The result should be similar to this:
If you zoom in, you will see that the white part is now smaller (with the stroke inside the shape), but you get the impression that the shadow is more even.
Hope this helps, even if a little!