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):

) {
    const lines = text.split(/\r\n|\r|\n/);
    const buf = [];
    const totalWidth = width - 2 * hPadding;
    for (const line of lines) {
      const chunks = this._splitLine(line, font, fontSize, totalWidth);
      for (const chunk of chunks) {
        const padding = buf.length === 0 ? hPadding : 0;
        buf.push(
          this._renderText(
            chunk,
            font,
            fontSize,
            width,
            alignment,
            padding,
            -fontSize 
          )
        );
      }
    }
    const renderedText = buf.join("\n");
    return (
      "/Tx BMC q BT " +
      defaultAppearance +
      ` 1 0 0 1 0 ${height} Tm ${renderedText}` +
      " ET Q EMC"
    );
  }
  _splitLine(line, font, fontSize, width) {