Your STL 3D model may have broken results in converters usually due to non-manifold geometry (like holes, overlapping faces, or inverted normals), as STL only stores surface triangles without topology information.
Non-manifold issues break the "watertight" structure converters need to map edge/face connections, causing glitches or incomplete outputs.
To fix this, try repairing the STL first with free tools like Blender or Meshmixer—use "Make Manifold" or "Repair Geometry" to fix common issues before converting.