"Preserve local edits" greyed out in linked text options

I am working on a guide made up of many chapters. Each chapter is intended to also function as a standalone document, so I have two alternate layouts, one with page numbering restarting in each section, the other with page numbering running sequentially through all the sections.

I have internal cross references within individual chapters, which I have amended in the layout functioning as a single continuous guide. See screenshots.

Chapter as standalone doc

Chapter as part of longer guide

I would like to exclude these when the linked copy is updated. I assume that the best way to do this is using the 'preserve local edits' function, but this is greyed out when I click into the link options:

Any suggestions for how to fix much appreciated!