@MikeTurner,
This is because the person menu generates different browser “profiles” when a new “user” is created. The newly created profile will have a different profile ID and therefore, a different set of bookmarks, browsing data and a different Sync code.
AFAIK, when you create a Sync chain, you create it based on the profile you’re currently “signed into”. Any additional profiles will not know about the Sync chain created initially. Let me know if this is unclear. cc @LaurenWags to confirm.