Modeling Algorithms - Boolean operation modifies a face by adding an internal edge instead of creating two faces

bsplinecurve spline_curve 1 2 0 2 9 2 -5 4 0 1 5 4 0 1

# It's strange that everything works correctly for this curve
# bsplinecurve spline_curve 1 2 0 2 9 2 -4 4 0 1 4 4 0 1

mkedge spline_edge spline_curve
prism spline_shell spline_edge 0 0 -5

plane cyl_plane 0 0 -1 0 1 0 0 0 1
pcylinder cyl_solid cyl_plane 1 5

setfillhistory 1
bclearobjects
bcleartools
baddobjects cyl_solid spline_shell
bapibuild bop_res
savehistory hist

checkshape bop_res

modified spline_shell_mod hist spline_shell

explode spline_shell_mod F
whatis spline_shell_mod

vinit
vsetdispmode 1
vdisplay spline_shell_mod
vfit