Updating the PathSegmentCollection inside of PathFigure does not cause it to re-render. Slightly tweaking a property of the container of the PathFigure (like the Margin) will cause the rerender.

Reproduction Xaml and C#:

<Path x:Name="SegmentPath"
        Fill="Green">
    <Path.Data>
        <PathGeometry>
            <PathGeometry.Figures>
                <PathFigureCollection>
                    <PathFigure x:Name="SegmentFigure"/>
                </PathFigureCollection>
            </PathGeometry.Figures>
        </PathGeometry>
    </Path.Data>
</Path>

private int count = 0;

private void RedrawHandler()
{
    count += 1;
    Draw();
}

private void Draw()
{
    PathFigure segmentFigure = FindName("SegmentFigure") as PathFigure;

    segmentFigure.Segments.Clear();

    // Create new Segment instances as needed and add them to segmentFigure.Segments

    // Work around, changing the left margin of the container such that it will always be different and force a re-render
    float leftMargin = (count % 10) * 0.0001f;
    Margin = new(leftMargin, 0, 0, 0);
}