Is the original 3D geometry some kind of cylinder without any sort of material in the middle? If so, it's possible the extra layer is being added due to issues with the STL file. I would recommend following some of the checks outlined on this page:
https://www.simplify3d.com/resources/ar ... sh-errors/
In particular, the different region repair modes at the bottom of that page can sometimes deal with problematic geometry to stop the issue.