Bug report: Rhino.Geometry.Curve.ProjectToPlane error with ArcCurve

I’ve got a wrong result when using Rhino.Geometry.Curve.ProjectToPlane on an ArcCurve.

Here is a Rhino file:
bug curve.3dm (41.1 KB)

Here is the script:

import Rhino
import scriptcontext as sc

def getCurrentCPlane():
    viewport = sc.doc.Views.ActiveView.MainViewport
    return viewport.GetConstructionPlane()

def getCurvesRefs():
    crvType = Rhino.DocObjects.ObjectType.Curve
    res = Rhino.Input.RhinoGet.GetMultipleObjects( "Select curves to flatten", False, crvType )
    if res[0] == Rhino.Commands.Result.Success:
        curvesRefs = []
        for objRef in res[1]:
            curvesRefs.append( objRef )
    return curvesRefs

def flattenCurves( curvesRefs ):
    if curvesRefs == None:
        return
    plane = getCurrentCPlane().Plane
    for curveRef in curvesRefs:
        curve = curveRef.Curve()
        curveOnPlane = Rhino.Geometry.Curve.ProjectToPlane( curve, plane )
        sc.doc.ActiveDoc.Objects.Replace( curveRef, curveOnPlane )
    sc.doc.ActiveDoc.Views.Redraw()

curvesRefs = getCurvesRefs()
flattenCurves( curvesRefs )

Steps to reproduce:

1. open the file, go to front view
   
2. run EditPythonScript, copy the script above.
3. run the script, select the curve and press enter.
4. the result is unexpected: it’s a circle normal to the plane, whereas it should be an ellipse.

I tested on several curves, it seems that it only happens when the curve is an ArcCurve.

Rhino 8 SR14 2024-12-10 (Rhino 8, 8.14.24345.15001, Git hash:master @ cdb74b46090a23fe242c51b288886f76467a741b)
License type: Commercial, build 2024-12-10
License details: Cloud Zoo

Windows 11 (10.0.22631 SR0.0) or greater (Physical RAM: 31GB)
.NET 7.0.0

Computer platform: LAPTOP  - Plugged in [100% battery remaining]

Hybrid graphics configuration.
  Primary display: AMD Radeon(TM) Graphics (AMD) Memory: 1GB, Driver date: 10-17-2023 (M-D-Y).
    > Integrated accelerated graphics device with 3 adapter port(s)
        - Secondary monitor is laptop's integrated screen or built-in port
        - Windows Main Display attached to adapter port #1
  Primary OpenGL: NVIDIA GeForce RTX 3070 Laptop GPU (NVidia) Memory: 8GB, Driver date: 10-15-2024 (M-D-Y). OpenGL Ver: 4.6.0 NVIDIA 566.03
    > Integrated accelerated graphics device with 4 adapter port(s)
        - Secondary monitor is laptop's integrated screen or built-in port

OpenGL Settings
  Safe mode: Off
  Use accelerated hardware modes: On
  GPU Tessellation is: On
  Redraw scene when viewports are exposed: On
  Graphics level being used: OpenGL 4.6 (primary GPU's maximum)
  
  Anti-alias mode: 4x
  Mip Map Filtering: Linear
  Anisotropic Filtering Mode: High
  
  Vendor Name: NVIDIA Corporation
  Render version: 4.6
  Shading Language: 4.60 NVIDIA
  Driver Date: 10-15-2024
  Driver Version: 32.0.15.6603
  Maximum Texture size: 32768 x 32768
  Z-Buffer depth: 24 bits
  Maximum Viewport size: 32768 x 32768
  Total Video Memory: 8 GB

Rhino plugins that do not ship with Rhino
  C:\Program Files\SimLab\Plugins\SimLab PDF From Rhino\SimLabPDFExporter.rhp	"SimLab PDF Exporter"	

Rhino plugins that ship with Rhino
  C:\Program Files\Rhino 8\Plug-ins\Commands.rhp	"Commands"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\rdk.rhp	"Renderer Development Kit"	
  C:\Program Files\Rhino 8\Plug-ins\RhinoRenderCycles.rhp	"Rhino Render"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\RhinoRender.rhp	"Legacy Rhino Render"	
  C:\Program Files\Rhino 8\Plug-ins\rdk_etoui.rhp	"RDK_EtoUI"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\NamedSnapshots.rhp	"Snapshots"	
  C:\Program Files\Rhino 8\Plug-ins\MeshCommands.rhp	"MeshCommands"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\IronPython\RhinoDLR_Python.rhp	"IronPython"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\RhinoCycles.rhp	"RhinoCycles"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\Toolbars\Toolbars.rhp	"Toolbars"	8.14.24345.15001
  C:\Program Files\Rhino 8\Plug-ins\3dxrhino.rhp	"3Dconnexion 3D Mouse"	
  C:\Program Files\Rhino 8\Plug-ins\Displacement.rhp	"Displacement"	
  C:\Program Files\Rhino 8\Plug-ins\SectionTools.rhp	"SectionTools"

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