Luban is filling embedded holes

I’m trying to make small magnets. I have some tiny 2mm x 3mm magnets and I’m trying to put them inside so children can’t get them. I put holes in my stl and a pause step, but the printer?/slicer? is just printing a cylinder inside. These cylinders were subtracted from the stl in Microsoft 3D Builder

I stopped this print. As you can see it just prints the cylinder

I’m not sure about Microsoft 3d builder as I use blender. I have the issue of polygons filling occasionally, and it is usually due to the faces being inside out! Not sure if that is/can be an issue here, but it could be worth looking into.

Just to follow up. It seems it’s Luban. It works if I slice it in Cura.