Problem with plotting a 3d Bezier curve

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This looks like a bug and I opened the ticket #31640 to track the issue. Thanks for your report!

The issue turns out to be a missing vector conversion in the code. I posted a fix and should hopefully make it for a later release.

As a work around you can use the following function (that contains the fix I provided in the ticket)

def bezier3d(path, **options):
    from . import parametric_plot3d as P3D
    from sage.modules.free_module_element import vector
    from sage.symbolic.ring import SR

    p0 = vector(path[0][-1])
    t = SR.var('t')
    if len(path[0]) > 2:
        B = (1-t)**3*vector(path[0][0])+3*t*(1-t)**2*vector(path[0][1])+3*t**2*(1-t)*vector(path[0][-2])+t**3*p0
        G = P3D.parametric_plot3d(list(B), (0, 1), **options)
    else:
        G = line3d([path[0][0], p0], **options)

    for curve in path[1:]:
        if len(curve) > 1:
            p1 = vector(curve[0])
            p2 = vector(curve[-2])
            p3 = vector(curve[-1])
            B = (1-t)**3*p0+3*t*(1-t)**2*p1+3*t**2*(1-t)*p2+t**3*p3
            G += P3D.parametric_plot3d(list(B), (0, 1), **options)
        else:
            G += line3d([p0,curve[0]], **options)
        p0 = vector(curve[-1])   # this was the wrong line!
    return G