Socket IO connection doesn't work on ios, but works on android (http 301 instead of 101)

[svbackend]

I have a simple Socket.io server for real time chat messages running as part of docker compose:

socketio:
 image: fairytrail/socketio:latest
 build:
 context: ./chat
 dockerfile: docker/Dockerfile
 env_file:
    - .env.local
 depends_on:
    - db
 restart: unless-stopped

it works locally (in expo go and in development build, both on ios and android) and it works on the server if I'm using android device, but it doesn't work on IOS (confirmed on Iphone 8, XR and 12):
I'm getting 301 http code response in gateway/nginx access logs and in the socket.on('connect_error'...) I'm seeing:
"Invalid Sec-WebSocket-Accept response.", which makes sense, since the expected response code is 101

it's deployed behind traefik which handles ssl certs and then request goes to my app nginx reverse proxy with the following config:

map $http_host $redirect_to {
 default spring.domain.app;
 www.spring.domain.app spring.domain.app;
}

server {
 listen 80 default_server;
 server_name spring.domain.app;
 server_tokens off;
 client_max_body_size 1M;

 gzip on;
 gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;

 error_log /var/log/nginx/error.log debug;
 access_log /var/log/nginx/access.log main;

 include /etc/nginx/snippets/resolver.conf;

 if ($redirect_to != $http_host) {
 # i was trying to comment out this line, it didn't help
 return 301 https://$redirect_to$request_uri;
 }

    location /socket.io {
 include /etc/nginx/snippets/cors.conf; # CORS headers
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Forwarded-Host $remote_addr;
 proxy_set_header Host $host;
 proxy_set_header Connection "upgrade";
 proxy_set_header Upgrade $http_upgrade;
 proxy_http_version 1.1;
 proxy_buffering off;
 proxy_pass http://socketio:3000/socket.io;
 }

    location / {
# ... more params
 include /etc/nginx/snippets/cors.conf;
 set $upstream http://api-gateway;
 }
}

And so I see in the access logs these lines for every connection attemts from IOS, as you can see response code is 301 (permanent redirect):
172.18.0.2 - - [21/Feb/2024:20:36:13 +0000] "GET /socket.io/?apiToken={secret}&EIO=4&transport=websocket HTTP/1.1" 301 162 "-" "-" "89.70.138.xxx"

While for android they are looking like this, response code is 101 and it works just fine:
172.18.0.2 - - [21/Feb/2024:20:50:41 +0000] "GET /socket.io/?apiToken={secret}&EIO=4&transport=websocket HTTP/1.1" 101 277 "-" "okhttp/4.9.2" "89.70.138.xxx"

What could cause such different behavior? Where do I even start for debugging it? Could it be bug in SocketIO client library? Or is it issue on behalf of "fetch" library on IOS/swift side? What can I try to debug it better?

[svbackend]

actually issue was indeed within these lines:

 if ($redirect_to != $http_host) {
 # i was trying to comment out this line, it didn't help
 return 301 https://$redirect_to$request_uri;
 }

maybe first time when i commented them out i forgot to restart nginx or something, I just double-checked - it fixed the issue.