Why Adorner Layer is not rendering properly on WPF TextBox?

I want to draw a wave-line pattern under particular text of a TextBox. But the pattern is not drawn properly in adorner layer.

Sample Link : https://drive.google.com/file/d/1g5148ydo961Hi0A3cA8G3eRnxn8v0arR/view?usp=sharing

In the below image you can see that, wave-line pattern is not rendered properly in many of the places. Highlighted in green are some of the properly rendered patterns.

If i drawn a simple line that will also not rendered properly. Notice the image below. Highlighted in green are some of the properly rendered lines.

protected override void OnRender(DrawingContext drawingContext)
    {
        TextBox textBox = this.AdornedElement as TextBox;
        //WordList stores the start index of a word and word as a dictionary collection.
        wordList.Clear();
        // SetRedUnderline method will provide  the wordList which needs to be underline.
        SetRedUnderline(textBox.Text);

        //pathPen is used to draw line with square cap at start and end.
        Pen pathPen = new Pen(new SolidColorBrush(Colors.Red), 0.2);
        pathPen.EndLineCap = PenLineCap.Square;
        pathPen.StartLineCap = PenLineCap.Square;
        //pathGeometry is used to draw segment of wave line by usig BezierSegment.
        Point pathStart = new Point(0, 1);
        BezierSegment pathSegment = new BezierSegment(new Point(1, 0), new Point(2, 2), new Point(3, 1), true);
        PathFigure pathFigure = new PathFigure(pathStart, new PathSegment[] { pathSegment }, false);
        PathGeometry pathGeometry = new PathGeometry(new PathFigure[] { pathFigure });
        //squigglyBrush is customized to darw a tile of pathGeometry. 
        DrawingBrush squigglyBrush = new DrawingBrush();
        squigglyBrush.Viewport = new Rect(0, 0, 6, 4.8);
        squigglyBrush.ViewportUnits = BrushMappingMode.Absolute;
        squigglyBrush.TileMode = TileMode.Tile;
        squigglyBrush.Drawing = new GeometryDrawing(null, pathPen, pathGeometry);
        foreach (KeyValuePair<int, object> keyValuePair in wordList)
        {
            Rect startPosition = textBox.GetRectFromCharacterIndex(keyValuePair.Key);
            Rect endPosition = textBox.GetRectFromCharacterIndex(keyValuePair.Key + keyValuePair.Value.ToString().Length - 1, true);
            Rect rectUnion = Rect.Union(startPosition, endPosition);
            drawingContext.DrawLine(new Pen(squigglyBrush,4), rectUnion.BottomLeft, rectUnion.BottomRight);
            //drawingContext.DrawLine(new Pen(Brushes.Red, 1), rectUnion.BottomLeft, rectUnion.BottomRight);
        }
    }

Thanks..!