That further adds to my suspicion that the model for that part is broken, and it is perhaps not a closed shape. If it’s missing e.g. the bottom surface that would explain it.
Is this a model you made, or downloaded? If you made it, try re-exporting it. If you downloaded it, open it in any model viewer (you can even import .stl’s into FreeCAD) and check it over for missing surfaces, edges that don’t touch, holes that shouldn’t be there, etc.
That further adds to my suspicion that the model for that part is broken, and it is perhaps not a closed shape. If it’s missing e.g. the bottom surface that would explain it.
Is this a model you made, or downloaded? If you made it, try re-exporting it. If you downloaded it, open it in any model viewer (you can even import .stl’s into FreeCAD) and check it over for missing surfaces, edges that don’t touch, holes that shouldn’t be there, etc.