I noticed that NPOI fixed a bug:

https://github.com/nissl-lab/npoi/commit/4e85b803eb29b1de2a85813e1d43baac32daaa81

The buggy code is as follows:

public void RemoveRelationship(String id)
{
  ...
  internalRelationshipsByTargetName.Values.Remove(rel);
...}

The fixed code is as follows:

public void RemoveRelationship(String id)
{
  ...
  internalRelationshipsByTargetName.RemoveAt(internalRelationshipsByTargetName.IndexOfValue(rel));
...}

NPOI programmers fixed a sortedlist remove error. POI still uses the buggy way:

 public void removeRelationship(String id) {
        PackageRelationship rel = relationshipsByID.get(id);
        if (rel != null) {
            relationshipsByID.remove(rel.getId());
            internalRelationshipsByTargetName.values().remove(rel);
        }
    }

It can have the identical error.