Structured FrameMaker 2019. Book Page Numbering update <$Sectionnum> gets stripped out

A Structured FM file uses Chapter and Section variables to create page numbering e.g 00-01-1, where 00 is the chapter variable, 01 is the section variable and the 1 is the page number. These have been set in a file master pages using Format > Document > Numbering.

When a book is created, that file is used to import the Element Definitions for the structure of the book. No problems so far.

When the file is added to the book and the book is updated the "sectionnum" variable gets stripped out leaving the numbering of the file as "00--1".

When selecting the file in the book and using Format > Document > Numbering to correct the numbering error, only the Chapter variable and Page are editable. The section and subsection edit area are grayed out?

What is the fix please? What should be set that would appear that has not been set

Thank you

Bob