Subtracting an extruded object from an OpenSCAD object or another extruded object seem to confuse the 'Preview' viewer. The models render correctly and show correctly in the 'Surfaces' viewer.
I am using version 1.1.1 with the latest extrude_along_path.py containing the fixes for issue #18
I added the function extrude_example_subtractions() to the attached path_extrude_example.py file. The problems show up most clearly if you rotate to look up the +y axis.
path_extrude_example.zip
7.