Attach (recommended) or Link to PDF file here:
[Testfile v1.5 to PDFA2.pdf](https://github.com/mozilla/pdf.js/files/6310752/Testfile.v1.5.to.PDFA2.pdf)


Configuration:
- Web browser and its version: Version 89.0.4389.114 (Offizieller Build) (x86_64)
- Operating system and its version: Windows 10 / MacOS BigSur
- PDF.js version: 2.5.207
- Is a browser extension: no 

Steps to reproduce the problem:
1.  Open Testfile v1.5 to PDFA2.pdf in pdf.js viewer demo
2. text is covered up by markings / highlights which are stored in pdf itself

What is the expected behavior? (add screenshot)
Should be displayed as in Acrobat Reader
<img width="1169" alt="Bildschirmfoto 2021-04-14 um 14 24 41" src="https://user-images.githubusercontent.com/1286150/114710202-bf175a80-9d2d-11eb-8d54-75481fe91b7d.png">

What went wrong? (add screenshot)
It's how it is displayed in pdf.js viewer
<img width="782" alt="Bildschirmfoto 2021-04-14 um 14 24 16" src="https://user-images.githubusercontent.com/1286150/114710311-d8200b80-9d2d-11eb-8000-b67b94488929.png">

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

parentBuffer = [`${this.parent.num} ${this.parent.gen} obj\n`];
        writeDict(parent, parentBuffer, parentTransform);
        parentBuffer.push("\nendobj\n");
      } else if (isDict(this.parent)) {
        this.parent.set("V", name);
      }
    }
    dict.set("AS", name);
    dict.set("M", `D:${getModificationDate()}`);
    let originalTransform = null;
    if (encrypt) {
      originalTransform = encrypt.createCipherTransform(
        this.ref.num,
        this.ref.gen
      );
    }
    const buffer = [`${this.ref.num} ${this.ref.gen} obj\n`];
    writeDict(dict, buffer, originalTransform);
    buffer.push("\nendobj\n");
    const newRefs = [{ ref: this.ref, data: buffer.join(""), xfa }];
    if (parentBuffer !== null) {
      newRefs.push({
        ref: this.parent,
        data: parentBuffer.join(""),
        xfa: null,
      });
    }
    return newRefs;
  }
  _processCheckBox(params) {