RE: weird slicing of an object
It could be a broken model. Is there a little warning triangle on the screen to the right of the platter view? If you post a saved project file (.3mf) maybe we can see what is going on. File->Save Project (or File ->Save Project As) to create a .3mf project file, then zip that up or the forum will silently reject it. Then post the zipped file here.
RE: weird slicing of an object
Read Sketchup, didn't need to know more. Almost guaranteed to be a broken model. As JimB said, you can upload the 3mf here to see if it can be fixed. If you see the warning triangle and you're on Windows, you can try to save it through the inbuilt netfabb tool. On other platforms, you can run the model through https://www.formware.co/onlinestlrepair and see if you can salvage it.
Formerly known on this forum as @fuchsr -- until all hell broke loose with the forum software...
RE:
If you modeled it yourself you know the dimensions for the part. Then it would be easy to change to another software that is better suited for this task, like Fusion 360 or Onshape. Both offer free licenses for makers/hobbyists and there are a lot of video tutorials for them on YT. To model a part like yours should not take very long in Fusion 360 and if you keep practising a few times you will get better very soon. Just don't wait a long time between two small projects, or you will forget too much and have to start from square one (or two). 🙂
RE: weird slicing of an object
I'm just a beginner, but have you tried in the Prusa Slicer, right click, "Fix through the Netfabb"? I was able to fix a model with "missing layers" this way.