You have to add the location /stream part to the conf file, by default the nginx proxy manager will recreate one without this part.
So, you need to ssh to the server it’s deployed and go to /proxy_manager/data/nginx/proxy_host/ and find the right *.conf file created for your service. Then vim or nano inside the file and copy paste the location stream part to it.
Finally you probably will have to restart the proxy, and watch out if the nginx proxy doesn’t recreate one file for the streamlit service. If so rename both files.