Word file used: 
[Formular_T.docx](https://github.com/mozilla/pdf.js/files/6907859/Formular_T.docx)

PDFs generated:
PDF 1: [Formular_T_default.pdf](https://github.com/mozilla/pdf.js/files/6907771/Formular_T_default.pdf)
PDF 2: [Formular_T_changed.pdf](https://github.com/mozilla/pdf.js/files/6907772/Formular_T_changed.pdf)

- PDF.js version: v2.9.359

Steps to reproduce the problem:
1. Create Word Document with Checkbox and check it.
2. Convert Word document to PDF using **Aspose.pdf** (for .net)

3. Extra: Change value of second Checkbox  in PDF 1 and save the file (PDF 2). The new File will show both Checkboxes checked in other browser, but only the newly checked one in PDF.js

Expected:
Checkbox is checked (pdf viewer from adobe/Chrome Version 92.0.4515.107 64-bit)
![image](https://user-images.githubusercontent.com/76941221/127659557-42be1cca-3c90-49b8-b224-70ee85aa1b30.png)

Result:
Checkbox is not checked (Mozilla Firefox/PDF.js)
![image](https://user-images.githubusercontent.com/76941221/127660064-e3cdc3f9-6887-4ae7-af27-d46cded69029.png)



const combWidth = (width / this.data.maxLen).toFixed(2);
    const buf = [];
    const positions = font.getCharPositions(text);
    for (const [start, end] of positions) {
      buf.push(`(${escapeString(text.substring(start, end))}) Tj`);
    }
    const renderedComb = buf.join(` ${combWidth} 0 Td `);
    return (
      "/Tx BMC q BT " +
      defaultAppearance +
      ` 1 0 0 1 ${hPadding} ${vPadding} Tm ${renderedComb}` +
      " ET Q EMC"
    );
  }
  _getMultilineAppearance(
    defaultAppearance,
    text,
    font,
    fontSize,
    width,
    height,
    alignment,
    hPadding,
    vPadding