Added/Missing Layers during Prusa Slicer Preview  

RE:

The reason is quite simple.  That model is full of errors.  When I load it into PS it shows as having 474 open edges and PS has 'fixed' 5922 other errors.  Look at the little yellow triangle next to its name on the right hand tree list.  Non manifold open edges are a killer for slicers.

Also even without that triangle warning, the upper areas and the tabs at the side are rendered in a darker green, this is a visual indication that something is wrong.  If you ever see that then expect slicing problems. 
If you are running windows the usual advice would be to use the right click and fix through netfabb option to repair it.  That often does manage a successful repair.  However for this model the netfabb repair (which is one of the best available) 'fixes' it in the same way that PS slices it and the model gets filled in.  Its good to show you exactly how the model is interpreted though due to the open edges.

I loaded the stl into Blender, (it showed the same number of errors in its 3dprint tool as PS did), and fixed it for you though (attached in the zip file).  If this is your own model then you need to learn how to generate valid manifold geometry in your design package if you want to 3d print.  If its not one you have designed then I'm afraid you are going to have to learn how to repair models found on the internet as there are so many broken ones out there on all the usual sites that just arent suitable for 3d printing/slicing with work.

RE: Added/Missing Layers during Prusa Slicer Preview

Hi Neophyl, thank you for fixing the model. It was one that I created myself. I'm quite surprised there were that many errors because on Sketchup it was just a few simple actions to create the shape - lesson learned though. I've used only some of the basics in Blender a couple of years back, so it might be worth continuing down that path and learning how to use it instead of Sketchup.

Regards, Ryan

RE: Added/Missing Layers during Prusa Slicer Preview

Not a Sketchup user myself but it's interesting that many reports of invalid geometries appear to come from Sketchup. For functional designs like this I use Fusion 360. Or if it's something simple like this one, TinkerCAD is a good choice. It's as easy to learn as can be but surprisingly powerful for simple, non-parametric designs.