Attach (recommended) or Link to PDF file here:
https://www.canada.ca/content/dam/ircc/migration/ircc/english/pdf/kits/forms/eimm5669e.pdf
[eimm5669e.pdf](https://github.com/mozilla/pdf.js/files/7225677/eimm5669e.2.pdf)

Configuration:
- Web browser and its version: Firefox nightly
- Operating system and its version: Linux Debian

Steps to reproduce the problem:
1. Look at the bottom of page 2

delete this[$extra];
    return result;
  }
}
class SubformSet extends XFAObject {
  constructor(attributes) {
    super(TEMPLATE_NS_ID, "subformSet",  true);
    this.id = attributes.id || "";
    this.name = attributes.name || "";
    this.relation = getStringOption(attributes.relation, [
      "ordered",
      "choice",
      "unordered",
    ]);
    this.relevant = getRelevant(attributes.relevant);
    this.use = attributes.use || "";
    this.usehref = attributes.usehref || "";
    this.bookend = null;
    this.break = null;
    this.desc = null;
    this.extras = null;
    this.occur = null;
    this.overflow = null;
    this.breakAfter = new XFAObjectArray();
    this.breakBefore = new XFAObjectArray();
    this.subform = new XFAObjectArray();
    this.subformSet = new XFAObjectArray();
    
  }
  *[$getContainedChildren]() {