I created this part and when I imported it into Cura for slicing and 3D printing, the two cylinders that were supposed to be hollow were filled.

I exported the .stl with default settings except I set Angle to 2°.



I threw the .stl into the first online stl fixer I could find (https://www.formware.co/onlinestlrepair) and it reported 3 Non-manifold edges and 6041 Inverted faces.

The repaired .stl sliced alright in Cura.