Did anyone fixed that in the meanwhile? I have the same Issue.

Testing with curl is running fine:
curl -i -N "http://localhost/?script=longtest.sh&path=test"

HTTP/1.1 200 OK
Server: nginx/1.26.3
Date: Sun, 18 May 2025 10:45:53 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Content-Encoding: identity
Cache-Control: no-cache, no-store

STEP 1 - 10:45:54
(PAUSE)
STEP 2 - 10:45:56
(PAUSE)
STEP 3 - 10:45:58
(PAUSE)
....

I tried serval configs. This is my current one:

server {
        listen       80;
        server_name  _;

        location / {
        # Buffering global deaktivieren
            fastcgi_buffering          off;
            fastcgi_request_buffering  off;
            fastcgi_max_temp_file_size 0;
            
            # Force Flush aktivieren
            #fastcgi_force_flush        on;

            # Chunked Encoding erzwingen
            chunked_transfer_encoding  on;
            proxy_buffering            off;
            gzip                       off;

            # Keep-Alive
            fastcgi_keep_conn          on;
            fastcgi_socket_keepalive   on;

            # Timeouts
            fastcgi_read_timeout       86400;
            fastcgi_send_timeout       86400;
        
#            autoindex on;
#            alias /mnt/samba/;
            # CGI für die dynamische Verzeichnisauflistung verwenden
            root /scans;
            try_files $uri $uri/ =404;  # Wenn die Datei nicht existiert, gehe zu @cgi
            # Wenn die Anfrage auf ein Verzeichnis zeigt, führe das CGI-Skript aus
            # Wenn es ein Verzeichnis ist, rufe das CGI-Skript auf
            location ~ /$ { 
                # Füge die FastCGI-Parameter hinzu
                include fastcgi_params;
                fastcgi_param SCRIPT_FILENAME /scans/scans.sh;
                # Erlaube Übergabe der Anfrage als Query-String
                #fastcgi_param QUERY_STRING "path=$uri";  # Statt $request_uri
                fastcgi_param QUERY_STRING $query_string; 
                fastcgi_param NO_BUFFERING 1;
                fastcgi_request_buffering off; 
                fastcgi_pass unix:/var/run/fcgiwrap/fcgiwrap.socket;
            }                    
            # Deaktiviere das automatische Directory-Listing von Nginx
            autoindex off;     

        }
    }

But It´s again buffering. That´s strange because the direct curl on the wrapper is not buffering.