Update here. I have discovered the problem. Was my code of course.
Sometime or other I had added into the Program.cs file the following
builder.Services.AddNewtonsoftJson();
According to GitHub Copilot what this does is -
• Configures the MVC/API controllers to use Newtonsoft.Json for serializing/deserializing JSON instead of the default System.Text.Json
And U now uses System.Text.Json everywhere so something was bound to conflict. In my case it showed up in the Schema Comparison table.
Removing that line fixed it. Yay.
I did attempt to mod my code to remove the use of Newtonsoft but it rapidly became very complicated so I want to put that off till this new version is stable.
Soldiering now.