Function response headers not preserved

**Describe the bug**

I am calling an Azure Function that runs inside the Azure Static Web App instance from a React frontend that also runs inside the Azure Static Web App instance. This function sets custom response headers but these headers do not show up in the response inside the frontend (the response headers seem to be overwritten somehow). This behaviour only occurs when I deploy the Static Web App to Azure. When run locally, everything works fine.

Stripped down function code:

```javascript
import { AzureFunction, Context, HttpRequest } from '@azure/functions'

const testFunction: AzureFunction = async function (context: Context, req: HttpRequest) {
...

context.res = {
status: 200,
headers: { 'content-type': 'application/json', 'content-range': 0-1/2 },
...
}
}

export default testFunction
```

I can see the header `content-type` but not the header `content-range`.

**Expected behavior**

The headers set inside the function should be preserved in the response.