In your case, you have 50 identical lines.
Move «List <Vector2> points = new List <Vector2> ();» inside the foreach.

foreach (SentimentMarker sentimentMarker in _sentimentMarkers)
{
    List<Vector2> points = new List<Vector2>();

Here's some sample code of mine. I'm only seeing only line shown on my map whereas there should be 50.

Cheers,

    private void DrawPersonalLinks()
    {
        List<Vector2> points = new List<Vector2>();

        foreach (SentimentMarker sentimentMarker in _sentimentMarkers)
        {
            points.Clear();
            points.Add(_yourLocationStaticMapMarker.position);
            points.Add(sentimentMarker.position);
               
            OnlineMapsDrawingLine line = new OnlineMapsDrawingLine(points, Color.white);

            // Draw the line route on the map.
            OnlineMaps.instance.AddDrawingElement(line);
        }
    }