When you add a new headword in a Personal Book, the old headword is supposed to close, so you only have one headword at a time. For some reason that's not working for me in one of my Personal Books:

What's also interesting is that this menu appears anywhere after the first headword (i.e. even before the second-fourth headwords appear in the text).
The following document is an extract from my PB that will reproduce the error: 0167.Headwords.docx