I'm trying to print a supporting base which will house the spindle for an electrostatic rotor. It's basically just a truncated cone with a hole down it's center to house the spindle.
For reasons that I cannot fathom, Ultimaker Cura keeps on adding an unrequested top/bottom layer (color-coded yellow in the screenshots) inside this hole, so instead of a single hollow cylinder of 10 mm depth, the result is a hole only a few millimeters deep with another hollow cylinder behind it.
Here is the intended model, note the open space for the hole at the top.
Here is the inner view of the hole being printed as expected:
Finally, here is a layer view of the print a few millimeters from the final top layer with the unrequested top/bottom layer that covers the spindle hole:
The STL file is on Github (with a built-in viewer).
Can anyone help me understand why this is happening?