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)

case "BM":
            this.ctx.globalCompositeOperation = value;
            break;
          case "SMask":
            if (this.current.activeSMask) {
              
              
              
              if (
                this.stateStack.length > 0 &&
                this.stateStack[this.stateStack.length - 1].activeSMask ===
                  this.current.activeSMask
              ) {
                this.suspendSMaskGroup();
              } else {
                this.endSMaskGroup();
              }
            }
            this.current.activeSMask = value ? this.tempSMask : null;
            if (this.current.activeSMask) {
              this.beginSMaskGroup();
            }
            this.tempSMask = null;
            break;
          case "TR":
            this.current.transferMaps = value;
        }
      }
    }
    beginSMaskGroup() {