I've uploaded the STL... I have checked it in multiple apps and it seems to be interpreted correctly.
I don't understand what you mean by "not a top" surface... I have included a screenshot from Cura 15 which gets the shell correct. While the 6 itself is not a top surface, the hole in the 6 IS a top surface. In fact, Cura knows it is because, if I enable ironing, it tries to iron that part but there is no shell to iron! I've included a sceengrab of the ironing layer with ironing enabled.
I am indeed slicing for a non-UM printer. Does that matter? I am slicing for an Anet A8. The STL is a contiguous shell but, as stated above, their should be some horizontal shell in the hole inside the 6. Instead, Cura just leaves this open (and you can see the infill). I've also included close-up of the printed 6 which has the shell inside the 6 missing. You can indeed see directly into the solid object.
UPDATE: Well, I've selected a different print profile and Cura get's it right! So, I guess one of my settings in my custom profile is causing this... any ideas which one? Can I send my my settings through?
I'll try to recreate the profile from the default 0.2mm layer height profile and see where it goes wrong,