Revolved spline surface missing in STEP export

I tried to create a revolved surface from a spline thickend by an offset and finally revolved. When I use the fluent API, the resulting surface is missing in the exported STEP file. Only the end caps will show, e.g. in FreeCAD (but also other software).

When I create a similar result with the direct API (though offset and revolve in reverse order) everything works as expected.

STL exports and previews work as expected for both variants and the submeshes seem to correctly share the topology on the interfaces.

MWE:

```
import cadquery as cq
import cadquery.func as cf

points = [(100, 10), (80, 50), (100, 100)]

workplane_spline = cq.Workplane("XY").spline(points).offset2D(2).revolve()
workplane_spline.export("workplane_spline.step") # misses the spline surfaces

freeapi_spline = cf.offset(
cf.revolve(
cf.spline(points), # points get extended with z=0
(0, 0, 0),
(0, 1, 0),
),
2,
)
cq.Workplane(freeapi_spline).export("freeapi_spline.step") # complete solid
```

**result of "workplane_spline"**

**result of freeapi_spline**