Why is Nginx truncating the JSON streaming response?

Our stack is Client(Browser) <-> Nginx Reverse Proxy <-> Webserver(Flask+Gunicorn) <-> Golang gRPC server

The problem is when the client makes a call to the /realtimedata endpoint, Flask then opens the gRPC connection and starts receiving data via a server->client unidirectional stream. It then passes it back to the client. When I run this without Nginx, I get all responses. When running with Nginx, some responses get truncated. For example, if we expect:

{
    "source": "serviceA",
    "timestamp": 123456789,
    "data": {
        "1": 24.55667,
        "2": -456.5656,
        ...
        "200": 5.678
    }
}

We get

{
    "source": "serviceA",
    "time

Then

        stamp": 123456789,
    "data": {
        "1": 24.55667,
        "2": -456.5656,
        ...
        "200": 5.678
    }
}

This would be printed in console.log. I have proxy_buffering off; in the nginx configuration otherwise no data makes it to the browser. Not sure how to resolve this issue.

Here is a minimum, reproducible example.

UPDATE: I've ran the minimum reproducible example with Apache2 instead of Nginx and experiencing the same random truncations.