Mystery solved. In attempting to register it manually per your request, and having to do a “Rebuild” in Visual Studio, it complained the server (IIS Express) was already running. After stopping IIS and running it again, it seemed your code worked. Curious, I removed your code and put it back to the way it was, manually stopped IIS Express and ran it again and low and behold it worked!
The take away seems to be the Appsettings.json file may not be reloaded upon debugging without first stopping IIS Express. This is inconsistent with actual code changes that ARE reflected on a debug without having to stop IIS Express. I wonder why that is?
Depends on what those code changes are, if you’re modifying Razor or Sharp Pages they can be reflected immediately without a restart but most changes to C# .cs require a restart unless you have Edit and Continue enabled - which in my Experience doesn’t work very well.