split plane center not as expected

In the following example, after the split, I expect the workplane to be in the plane of the split, but it looks like it is centered on the center of gravity of the remaining part, even if I call a `workplane(centerOption="ProjectedOrigin")`:

```
import cadquery as cq
from math import pi as pi, sin as sin, cos as cos, tan as tan, atan as atan
col_limon = (230, 153, 0, 0.5)

result0 = cq.Workplane("XY" ).box(100, 40, 20)
result = result0.faces(">Z").workplane().transformed(rotate=(90, 20, 0)).split(keepTop=True).workplane(centerOption="ProjectedOrigin").circle(20).extrude(20, combine = True)
show_object(result0, options=dict(rgba=col_limon))
show_object(result)
```