Unable to explode this to separate inside and outside large surfaces

Having had an object refuse to explode and separate out its inner and outer surfaces, I try for a segment of it.
Still the inner and outer main surfaces are bound along the one edge (small filletted edge) after exploding this segment.

I am trying to extract just the outer main surface with holes in and delete all radiused edges etc.
ExtractSrf also refuses to extract just the outer face.

Why ?
see attached.

Unable to explode fully.3dm (132.3 KB)

Run DivideAlongCreases SplitAtTangents=Yes and then it should explode more or less as you want. You do have one fillet surface where it seems to be curvature continuous and not just tangent continuous, so the above won’t get that location. You will have to split that manually with Split>Isocurve.

HTH, --Mitch