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)

const { context: ctx } = this.cachedCanvases.getCanvas(
        "isFontSubpixelAAEnabled",
        10,
        10
      );
      ctx.scale(1.5, 1);
      ctx.fillText("I", 0, 10);
      const data = ctx.getImageData(0, 0, 10, 10).data;
      let enabled = false;
      for (let i = 3; i < data.length; i += 4) {
        if (data[i] > 0 && data[i] < 255) {
          enabled = true;
          break;
        }
      }
      return shadow(this, "isFontSubpixelAAEnabled", enabled);
    }
    showText(glyphs) {
      const current = this.current;
      const font = current.font;
      if (font.isType3Font) {
        return this.showType3Text(glyphs);
      }
      const fontSize = current.fontSize;
      if (fontSize === 0) {