Help topics won't open when deployed locally or on network drive

[gilloxygen]

Oxygen 20.1
Webhelp Classic
Chrome version 129.0.6668.101
I normally generate my help output locally to my C: drive or to a drive on the company network to test it before I deploy to the application's QA environment which is on a server somewhere. I then right-click the index.html file and click Open with > Chrome. The help opens with the default (first) topic displayed and the help works normally in that I click a book to display the parent topic content, or click a child topic to display the topic content. Simple!
Not today! When I click a book icon in the TOC the sub-topics are displayed, but when I click a topic it is displayed briefly and then the display reverts back to the default page (the first topic in the TOC). So I cannot see any topic page except for the first page in the ditamap.
When I deploy it to the QA environment (on another server I believe), the help works correctly either accessed via URL to the topic's deploy location or by accessing it within the application.
I get the same issue when opening the help on my C drive or the network drive in MS Edge, and colleagues have the same issue when they try their projects, so it is not specific to me or Chrome
Does anyone know what this is due to? Is it browser security related or Oxygen related? Can I resolve this so I can test without having to deploy to the QA environment on the server?