Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Chrome 90.0.4430.93
- Operating system and its version: macOs Big Sur 11.2.3
- PDF.js version: 2.7.570
- Is a browser extension: No

Steps to reproduce the problem:
1. Load the attached pdf in https://mozilla.github.io/pdf.js/web/viewer.html
2. Go to page 46. Dotted lines are missing in the graphics (see screenshot from acrobat reader)

What is the expected behavior? (add screenshot) See the screenshot
[Technical_Guide_UK.pdf](https://github.com/mozilla/pdf.js/files/6413001/Technical_Guide_UK.pdf)
![Screenshot 2021-05-03 at 07 46 09](https://user-images.githubusercontent.com/3785784/116844541-a343fd80-abe3-11eb-94c3-d03b3dfd6769.png)

colors.push(newColor);
      }
    }
    figureCoords[0] = pi[0];
    figureColors[0] = ci[0];
    figureCoords[splitXBy] = pi[3];
    figureColors[splitXBy] = ci[1];
    figureCoords[verticesPerRow * splitYBy] = pi[12];
    figureColors[verticesPerRow * splitYBy] = ci[2];
    figureCoords[verticesPerRow * splitYBy + splitXBy] = pi[15];
    figureColors[verticesPerRow * splitYBy + splitXBy] = ci[3];
    mesh.figures[index] = {
      type: "lattice",
      coords: figureCoords,
      colors: figureColors,
      verticesPerRow,
    };
  }
  function decodeType6Shading(mesh, reader) {
    
    const coords = mesh.coords;
    const colors = mesh.colors;
    const ps = new Int32Array(16); 
    const cs = new Int32Array(4); 
    while (reader.hasData) {
      const f = reader.readFlag();
      if (!(0 <= f && f <= 3)) {
        throw new FormatError("Unknown type6 flag");
      }
      const pi = coords.length;