Why is this central area of the STL being filled in?

Question

We've been doing some work automatically generating STL files using Python. We've made a ring of cubes like so:

Importing it into Cura still makes it look valid, with a hole in the center:

However, post slicing it comes up with saying it will take 14 hours to print! A ludicrous time. Looking at the preview, it seems to be adding supports to the entire inside of the structure:

And also a top / bottom layer:

Why is this? You can tell it's adding supports to the center and is not filled in due to the differing structure midway through the block:

Answer 1

The surfaces are clearly ill defined - it shows the top surface as a bottom one (those are marked red), and so the normals of that surface are pointing to the wrong direction.