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)

offsetY,
          scaleX,
          scaleY,
          subtype: group.smask.subtype,
          backdrop: group.smask.backdrop,
          transferMap: group.smask.transferMap || null,
          startTransformInverse: null, 
        });
      } else {
        
        
        currentCtx.setTransform(1, 0, 0, 1, 0, 0);
        currentCtx.translate(offsetX, offsetY);
        currentCtx.scale(scaleX, scaleY);
      }
      
      
      copyCtxState(currentCtx, groupCtx);
      this.ctx = groupCtx;
      this.setGState([
        ["BM", "source-over"],
        ["ca", 1],
        ["CA", 1],
      ]);
      this.groupStack.push(currentCtx);
      this.groupLevel++;
      
      this.current.activeSMask = null;
    }
    endGroup(group) {