Fetch fails when exceeding a certain execution time even-though using proper CORS headers

I have been stuck with a weird issue with CORS for quite some time now. To explain the setup I have a frontend JS snippet (including fetch API call to my own server) which I want to use as an embed code snippet other web applications can use. (ex: kind of like google analytics)

// JS snippet to copy to an unknown website 

<script>
// extract data and add to the body
const extracted_data = {}

fetch("https://api.xxxx.com/xxxxxx/create", {
        method: "POST",
        mode: 'cors',
        body: JSON.stringify(extracted_data),
        referrer: "origin",
        headers: {
            'Content-Type': 'application/json',
        },
    })
        .then(function (response) {
            // The API call was successful!
            if (response.ok) {
                return response.json();
            } else {
                return Promise.reject(response);
            }
        })
        .then(function (data) {
            // This is the JSON from our response
            console.warn("Successfull!", data);
            alert("Success:" + JSON.stringify(data));
        })
        .catch(function (err) {
            // There was an error
            console.warn("Something went wrong ->", err);
            alert("error:" + err.message);
        });

</script>

The problem is even if I have set my fetch API as below and the correct CORS headers are in my preflight response from my API it works only when the API call resolves immediately. If the API takes more time Fetch throws this common error even if the preflight is successful.

TypeError: Failed to fetch

I verified it using adding the below code to my API. Then it stops working and throws the above error. When I don't have any time taking functions inside my API call it works without errors.

 // 10 sec delay to check async behavior of the API
 await new Promise(resolve => setTimeout(resolve, 10000)); 

Any recommendations on how I should proceed to resolve this?

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