good catch! For some reason the embedded support was indeed missing for part cube_1hole_open.stl which is surprising because the OpenSCAD source (managed on GitHub) had been correct. I just updated the part here and also for the tractor and the forklift. Thank you for pointing me to this issue!
Btw., I am glad that you like kbricks and I am curious to see what you will build from it. Please let me know if you have any recommendations for improvement!