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)

while (this.stateStack.length || this.current.activeSMask !== null) {
        this.restore();
      }
      this.ctx.restore();
      if (this.transparentCanvas) {
        this.ctx = this.compositeCtx;
        this.ctx.save();
        this.ctx.setTransform(1, 0, 0, 1, 0, 0); 
        this.ctx.drawImage(this.transparentCanvas, 0, 0);
        this.ctx.restore();
        this.transparentCanvas = null;
      }
      this.cachedCanvases.clear();
      if (this.imageLayer) {
        this.imageLayer.endLayout();
      }
    }
    
    setLineWidth(width) {
      this.current.lineWidth = width;
      this.ctx.lineWidth = width;
    }
    setLineCap(style) {