Fetch API stream doesn't work on server but does locally

Question

As I explained in a previous question I have a FastAPI endpoint that returns a StreamingResponse response that is then consumed by a React application using fetch().body.getReader() API.

The problem I'm facing appears when I open my React application, select the image (s) using Uppy and send it to my FastAPI endpoint, locally it works just fine and the images are returned as a stream response:

But when I deploy my application on Heroku or Render the rendered response is all broken:

Adding more context to my previous question I'm rendering the stream using an async generator:

async function* submit({data}) {
    const formData = new FormData()

    data?.current?.files?.successful.map(image =>
        formData.append('images', image.data)
    )

    formData.append('language', data.current.language.code)

    try {
        const response = await fetch(
            'endpoint',
            {
                method: 'POST',
                body: formData
            }
        )

        const reader = response.body.getReader()

        while (true) {
            const { value, done } = await reader.read()

            if (done) break

            const base64 = `data:${response.headers.get(
                'content-type'
            )};base64,${btoa(String.fromCharCode(...new Uint8Array(value)))}`

            yield base64
        }
    } catch (error) {
        // ...
    }
}

That is called when the "Gallery" component in the screenshot is rendered:

const [images, setImages] = useState([])

    useEffect(() => {
        ;(async () => {
            for await (const image of await translateSubmit({
                data
            })) {
                setImages(previous => [...previous, image])
            }
        })()

        // eslint-disable-next-line
    }, [])

I was expecting to get the same result in the server when the application is deployed, but it just doesn't work as it should and I'm not sure how to approach the problem. Any tips?