Blazor Code-Behind: Why Is OnInitialized Not Finding Its Overridden Method?

You should remove the line

@inherits CreateForecast

I checked, adding @inherits does produce:

Error CS0115 'FetchData.BuildRenderTree(RenderTreeBuilder)': no suitable method found to override...

That was not obvious.

@inherits was used in older versions of Blazor when the classes were not yet generated as partial.

Now they are partial and you are trying to inherit a class from itself.

Side note, in CreateForecast.Razor.cs you can shorten the defintion to

//public partial class CreateForecast : ComponentBase
  partial class CreateForecast 

this reduces clutter and the chance for errors.