The issue was caused by the order of libraries given to the linker. More specifically, libbf_python.a should precede libbf_freestyle.a, which is assured by choosing an appropriate 'priority' value of a BlenderLib.
The issue was caused by the order of libraries given to the linker. More specifically, libbf_python.a should precede libbf_freestyle.a, which is assured by choosing an appropriate 'priority' value of a BlenderLib.