Surfaces get their 3D position solely from the 3D positions of their defining objects. So if one of the objects (a point, line, curve or edge) is not 3D, the Surface will not include that object in its 3D shape calculation. When the number of objects, with valid 3D shape, fall below the minimum needed to define a particular surface type then that surface will no longer be 3D (no display in the 3D Viewer or projections and will not export but will show up in the Surface Table still).