Cavities in objects disappear in PrusaSlicer  

Cavities in objects disappear in PrusaSlicer

I'm using PrusaSlicer 2.72+macOS and I'm having cavities inside a print object disappear when it's sliced. I'm making my objects in Blender, using the 3D Print Tool to check the mesh to be sure it's manifold, and on import to PrusaSlicer, I am getting no error messages.

I'm using a designed based on one from @Neophyl that he posted in some other topics when I asked about using screws with printed objects. Here's a view of the cutout I use to make space in an object for a screw and a square nut:

The red arrows point to gaps in the cutout, allowing the slicer to create a single layer between the slot for the nut and the hole for the screw. The larger round area at the top is a recess for the screw head. Again, a gap for a layer, since that makes the bridging across the empty areas work better when printing. (And, of course, I can easily adjust the length of the nut slot and the length of the screw holes as needed.)

I want some extra space for the end of the screw, so I don't have to have the cutout an exact length, so when the screw is tightened, some of the end has room to stick out beyond the nut. (And right now I have a mix of 20mm and 25mm screws, but not enough 20mm, so I allowed the extra room so I could use either length in this.) I'm using these upside down, so they go into a part like this:

The hole for the end of the screw, in the red box in this image, does not connect to the outside surface of my print object, so it forms a cavity inside the object I'm printing. When I use PrusaSlicer to slice this object, that section of the screw hole, which is basically a cavity totally enclosed and surrounded by the print object, disappears. I'm guessing, since it has no exposure or connection to the outside surfaces, PrusaSlicer sees it as unnecessary. It vanishes so all that's left of the screw hole is this:

So all of the hole meant for any extra part of the screw sticking out beyond the nut it goes into is now gone.

I tried several different ideas that didn't help, so I finally made a tiny "air hole" that went from an unseen part of the model into the hole for the screw. There are six of these in this object, each in a brace/support in the cone. They're highlighted in orange:

In each of these supports, you can see the screw hole coming up from the bottom, then the nut slot, then the top part of the screw hole. Coming out, about midway up, in that upper part of the hole, you can see a tiny (I think about 2 layers high) tube going out to the edge of the support it's in. The only purpose for these holes is to connect what PrusaSlicer sees as a cavity to the exterior of the model, so I don't care if that "air vent" tube comes out of a print all plugged up or not. In fact, I considered making it so small that PrusaSlicer wouldn't even include it, but I didn't want to see if, once that tube was gone, the software ignored the rest of the cavity, too.

This is one of those times I'm not even sure if I'm using the correct terms to describe things like the cavity. Is this a known issue or something I can fix by changing a setting in PrusaSlicer? Or is this something that would normally be handled in some known and accepted way?