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):

if (blendMode) {
      gsDict.set("BM", Name.get(blendMode));
    }
    const stateDict = new Dict(xref);
    stateDict.set("GS0", gsDict);
    const resources = new Dict(xref);
    resources.set("ExtGState", stateDict);
    resources.set("XObject", formDict);
    const appearanceDict = new Dict(xref);
    appearanceDict.set("Resources", resources);
    const bbox = (this.data.rect = [minX, minY, maxX, maxY]);
    appearanceDict.set("BBox", bbox);
    this.appearance = new StringStream("/GS0 gs /Fm0 Do");
    this.appearance.dict = appearanceDict;
    
    
    this._streams.push(this.appearance, appearanceStream);
  }
}
class WidgetAnnotation extends Annotation {
  constructor(params) {
    super(params);
    const dict = params.dict;
    const data = this.data;
    this.ref = params.ref;
    data.annotationType = AnnotationType.WIDGET;
    if (data.fieldName === undefined) {
      data.fieldName = this._constructFieldName(dict);
    }
    if (data.actions === undefined) {