Something has changed about the way support is automatically generated.
I have parts with voids modeled into them that in the past support material has been added to automatically. The voids are there primarily to control stresses and warping.
Previous versions had no problems with this. The factory file I attached 25Cube_4 is an open cube (4 sided) where support material is generated just fine. The file 25Cube_6 is a six sided hollow cube that no support material seems to be generated for.
This is really frustrating, as I'm trying to slice/print a new version of a part I have printed many times before, and suddenly this little thing doesn't work....