Mesh Vertices Shape Inconsistency

[Mon-ius]

Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for main branch).

Describe the issue

The Vertices shape is not consist before and after o3d.io.write_triangle_mesh. The original data store in *.ply, and after convert to *.stl, the mesh.vertices shape changed, but mesh.triangles remains the same.

Steps to reproduce the bug

def export(self, pth: str) -> str:
        if not os.path.exists(pth):
            os.mkdir(pth)
        
        data_in = self.input.data()
        data_out = self.output.data()
        
        points_in = np.asarray(data_in.vertices)
        points_out = np.asarray(data_out.vertices)
        
        faces_in = np.asarray(data_in.triangles)
        faces_out = np.asarray(data_out.triangles)
        
        print(f"Before, <{points_in.shape} - {faces_in.shape}> || <{points_out.shape} - {faces_out.shape}>")

        # o3d.io.write_triangle_mesh(os.path.join(pth, "input.ply"), data_in)
        # o3d.io.write_triangle_mesh(os.path.join(pth, "output.ply"), data_out)

        # data_in = o3d.io.read_triangle_mesh(os.path.join(pth, "input.ply"))
        # data_out = o3d.io.read_triangle_mesh(os.path.join(pth, "output.ply"))
        o3d.io.write_triangle_mesh(os.path.join(pth, "input.stl"), data_in)
        o3d.io.write_triangle_mesh(os.path.join(pth, "output.stl"), data_out)

        data_in = o3d.io.read_triangle_mesh(os.path.join(pth, "input.stl"))
        data_out = o3d.io.read_triangle_mesh(os.path.join(pth, "output.stl"))

        points_in = np.asarray(data_in.vertices)
        points_out = np.asarray(data_out.vertices)
        
        faces_in = np.asarray(data_in.triangles)
        faces_out = np.asarray(data_out.triangles)
        print(f"After, <{points_in.shape} - {faces_in.shape}> || <{points_out.shape} - {faces_out.shape}>")

        return pth

Error message

No response

Expected behavior

No response

Open3D, Python and System information

- Operating system: Ubuntu 22.04
- Python version: Python 3.12
- Open3D version: `0.18.0+a38ec05`
- System architecture: x86
- Is this a remote workstation?: yes
- How did you install Open3D?: conda

Additional information

For example,

        o3d.io.write_triangle_mesh(os.path.join(pth, "input.stl"), data_in)
        o3d.io.write_triangle_mesh(os.path.join(pth, "output.stl"), data_out)

        data_in = o3d.io.read_triangle_mesh(os.path.join(pth, "input.stl"))
        data_out = o3d.io.read_triangle_mesh(os.path.join(pth, "output.stl"))

the output is,

Before, <(512, 3) - (885, 3)> || <(508, 3) - (903, 3)>
After, <(2655, 3) - (885, 3)> || <(2709, 3) - (903, 3)>

While,

        o3d.io.write_triangle_mesh(os.path.join(pth, "input.ply"), data_in)
        o3d.io.write_triangle_mesh(os.path.join(pth, "output.ply"), data_out)

        data_in = o3d.io.read_triangle_mesh(os.path.join(pth, "input.ply"))
        data_out = o3d.io.read_triangle_mesh(os.path.join(pth, "output.ply"))

the output is,

Before, <(512, 3) - (885, 3)> || <(508, 3) - (903, 3)>
After, <(512, 3) - (885, 3)> || <(508, 3) - (903, 3)>

No response