UpdateSubstitutePart Not Working Correctly

[ThomasRambach]

I have this code:

                        if(PRTdoc.IsSubstitutePart)
                        {
                            bool boolUpdateSub = PRTdoc.UpdateSubstitutePart(false); 

                            if (boolUpdateSub == false)
                            {
                                // Now you done messed up
                            }
                        }

I thought it was fairly straight forward, but most of the time when it saves the IPT substitute, when you re-open the file, the link to the parent IAM is broken until you manually update the substitute. Am I missing something here?