Why does slicer keep adding this top infill when there is no geometry in my file?
It is properly caused by an error in the STL model. Try running a repair on the model - If on Windows right-click the model and choose repair.
When in Expert mode there will be a warning triangle to the left of the model in the list if it has errors.
When I fixed the model, it added the unwanted flat part.
I figured it out: Inside my model in Rhino, there was a open polysurface that was being exported with the model, and then Slicer filled it when slicing the model. Thanks for your help!
Thanks for the Update Andrew