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 activeSMask = this.current.activeSMask;
      const drawnWidth = activeSMask.canvas.width;
      const drawnHeight = activeSMask.canvas.height;
      const cacheId = "smaskGroupAt" + this.groupLevel;
      const scratchCanvas = this.cachedCanvases.getCanvas(
        cacheId,
        drawnWidth,
        drawnHeight,
        true
      );
      const currentCtx = this.ctx;
      const currentTransform = currentCtx.mozCurrentTransform;
      this.ctx.save();
      const groupCtx = scratchCanvas.context;
      groupCtx.scale(1 / activeSMask.scaleX, 1 / activeSMask.scaleY);
      groupCtx.translate(-activeSMask.offsetX, -activeSMask.offsetY);
      groupCtx.transform.apply(groupCtx, currentTransform);
      activeSMask.startTransformInverse = groupCtx.mozCurrentTransformInverse;
      copyCtxState(currentCtx, groupCtx);
      this.ctx = groupCtx;
      this.setGState([
        ["BM", "source-over"],
        ["ca", 1],
        ["CA", 1],
      ]);
      this.groupStack.push(currentCtx);
      this.groupLevel++;
    }
    suspendSMaskGroup() {