Dependency Injection not instantiating instances when using function.json rather than attributes

[burtonr]

I've created an Azure Function in C#, and attempting to utilize the dependency injection feature while also defining the Function parameters with a function.json file rather than the .net attributes so that I can use route parameters in the Function methods as described in Customize the HTTP Endpoint.

When I use the HttpTrigger and FunctionName attributes, the dependency injection works as expected. However, when I remove the attributes, and manually define the function.json file, the runtime throws an exception:

System.Private.CoreLib: Exception while executing function: MyClass.myFunction. Anonymously Hosted DynamicMethods Assembly: Object reference not set to an instance of an object.

To be sure, I've copied the automatically generated function.json file when using the attributes in order to ensure I hadn't miss-typed a value somewhere (removed the configurationSource field and ensured my written function.json file is in the output directory)

Versions and other information:
.csproj file sample:

<PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <AzureFunctionsVersion>v3</AzureFunctionsVersion>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.Azure.Functions.Extensions" Version="1.1.0" />
    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.16" />
    <PackageReference Include="Microsoft.Extensions.Http" Version="3.1.16" />
    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="3.0.13" />
  </ItemGroup>

Functions Core Tools:

$ func --version
3.0.3331

I haven't seen a sample of using dependency injection with route parameters, so I may have missed something in the setup. My Startup.cs file matches the example in the Dependency Injection docs.

Happy to provide any other information, or a reproducible example repo if needed.