fastapi streaming response is not decoded in chunks with compression enabled

Link to the code that reproduces this issue

https://github.com/HP2706/nextjs_streaming_bug

To Reproduce

to reproduce run npm run dev from root of the project
preferably use an virtual environment with Python 3.9.18

next click on the "stream" button
about 10 seconds later the number 1 .. 10 will appear on the screen
next to next.config.mjs and uncomment compression : false
now reload and click again.
this time the numbers will actually be stream as expected from the fastapi endpoint

Current vs. Expected behavior

i would expect the client to receive the streamingResponse from fastapi in chunks and not all at once even even without explicitly disabling compression

Provide environment information

Node : v21.5.0
os : darwin 23.2.0
Python : 3.9.18
fastapi : 0.100.1
uvicorn :0.23.2

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Vercel (Deployed)

Additional context

No response

Start with the linked reproduction repository and its npm run dev flow; compare streaming with compression enabled against next.config.mjs with compression: false. Check the FastAPI endpoint and the affected Next.js stages listed in the issue. Done means the client receives the response in chunks while compression remains enabled.