The compartment forming works by Boolean operations on closed contours. The compartment is a series of sections calculated at intervals along the length of the compartment. At each longitudinal position, up to three contours are formed and intersected to give the compartment section:
- Hull contour
- Box contour (from the top; bottom; port; stbd extents, interpolated if necessary)
- Boundary surface contour (optional).
With an unclosed contour, e.g. a line, it is not possible to determine which part of the hull contour defined the compartment.
The problem you are experiencing is due to the boundary surfaces not defining any area – they define just a flat line.