This has happened to me many times:
I have an open quilt and want to round a corner on it. The corner is where 2 surfaces meet. These surfaces have boundaries that lie on a plane. Where the round comes down to this plane, instead of lying on the plane, it kind of lifts up and away from the plane. Usually I have to mirror the quilt and merge the 2 quilts (to create a closed quilt, then a solid), because the thing is symmetrical. But of course, when I merge the two halves they don't close because of that round edge not lying on the plane.
Does anyone know a way of making it behave? I understand the geometrical reason why it does that... the trouble is I can't imagine a scenario where an engineer wants the edge of his radiused corner to lift away in a graceful spline tangential to the adjecent edges. Flat part lines are cheaper.
And no, I don't want to make it a closed quilt with a flat surface on the plane, for reasons I can't be bothered explaining as it's going-home time 
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- Extend the surfaces and trim the extra after the round.
- Sweep the round along two guide curves on the surfaces and merge.
- Define the surfaces with the round as part of it.
- Extend the round and trim it.
What you are seeing is as expected.
When you start working with draft angles, 3D turns into 12D 
