You have an stl file with ERRORS in it!....and S3D is not able to correctly identify what is what in the shape. And this is NOT a software bug, btw!
If you use Windows 10, load the stl into the free Windows app 3DBuilder. This app will auto repair the file. Export as a .3mf file, and losd the .3mf file into S3D.
If you can't use 3DBuilder, you'll need to use one of the stl repair services on the net.