Did not recieve 'That's not how physics works' acheivement

I created a craft in career mode via a command-disk with the bottom shape edited to have a width of 0, and a depth of 1; and the top shape to have a width of 1, and a depth of 0. The behaves like it has no mass, and is reported as such in both the part-menu in flight, as well as in the craft overview in the designer. However, I did not receive the hidden achievement. Of note - the craft overview in flight did not list the craft as massless, but instead listed it as having a mass of 500g.