Your local environment is running in-process, but is your Azure environment running dotnet-isolated by chance? I was getting an empty request body in my function after I migrated it from in-process model to isolated worker model. After some research I modified my function definition and return object to match what the isolated model likes, then it worked. Changes were:

• in the function definition I replaced the request and response object types: IActionResult changed to HttpResponseData and HttpRequest to HttpRequestData
• I changed the returned object type from IActionResult to HttpResponseData