CORS error despite CORS being enabled in app server and Nginx

Question

I have a web application and server which is fully functional in a debugging environment, however when it's hosted for production, I get the following error in the browser console:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://api.mydomain.com/api/search. (Reason: CORS request did not succeed)

The client is a Javascript app (accessible via mydomain.com) which makes fetch requests to the API server (Flask application, accessible via api.mydomain.com)

The javascript fetch request has the following format:

fetch(url, {
        method: "POST",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify(data),
        mode: "cors"
    })

The flask server has CORS enabled with:

CORS(app, resources={r"/*": {"origins": "*"}})
app.config['CORS_HEADERS'] = 'Content-Type'

This is the Nginx config:

server {
    listen 443 ssl;
    listen [::]:443 ssl;

    server_name mydomain.com api.mydomain.com www.mydomain.com;

    location /api/ {
        include proxy_params;
        proxy_pass http://unix:/home/adminuser/mydomain_server/mydomain/server/mydomain.sock;

        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
    }

    location / {

        # CORS
        if ($request_method = 'OPTIONS') {
            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            #
            # Custom headers and headers various browsers *should* be OK with but aren't
            #
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
            #
            # Tell client that this pre-flight info is valid for 20 days
            #
            add_header 'Access-Control-Max-Age' 1728000;
            add_header 'Content-Type' 'text/plain; charset=utf-8';
            add_header 'Content-Length' 0;
            return 204;
        }
        if ($request_method = 'POST') {
            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
            add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range';
        }
        if ($request_method = 'GET') {
            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
            add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range';
        }

        proxy_pass http://127.0.0.1:3030;

        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;

    }


    ssl_certificate /etc/letsencrypt/live/mydomain.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/mydomain.com/privkey.pem;
}

server {
    listen 80;
    listen [::]:80;

    server_name mydomain.com api.mydomain.com www.mydomain.com;

    return 302 https://$server_name$request_uri;
}

The strange thing is that one of the GET requests works without issue, another GET request initially fails with CORS error but works after a delay. I cannot work out what the issue is.