When the boolean-difference section of the sample is face up, it can be seen within the slicer-preview. However, if you flip the model 180 degrees so the cutout is facedown, the cutout can't be seen at all.
The attached sample was used instead of my actual model because it's extremely basic, and watertight. It also rules out possible causes such as
- walls are too thin
- boolean difference too small to slice
Even with this most-basic model, I can't seem to get Cura to acknowledge boolean-differences, when face down.