Tax-Provider for app isn't working

I’m trying to build a tax provider using the app system according to the docs:

<tax>
        <tax-provider>
            <!-- Unique identifier of the tax provider -->
            <identifier>myCustomTaxProvider</identifier>
            <!-- Display name of the tax provider -->
            <name>My custom tax provider</name>
            <!-- Priority of the tax provider - can be changed in the administration as well -->
            <priority>1</priority>
            <!-- Url of your implementation - is called during checkout to provide taxes -->
            <process-url>http://localhost:8000/shopware/tax</process-url>
        </tax-provider>
    </tax>

Here, the process url gets called during final checkout page, I get all the data to perform tax calculation. Then at the end I return the response as below - [Mentioned on the docs].
I’ve hardcoded the response just to see if the taxes are getting injected. But it isn’t getting injected.
Tax estimation requests are coming in, I’m processing and providing the response but the returned taxes aren’t getting injected into the cart.

dummy_data = {
        "lineItemTaxes": {
            item["uniqueIdentifier"]: [
                {
                    "tax": item["price"]["totalPrice"] * 0.25,
                    "taxRate": 25.0,
                    "price": item["price"]["totalPrice"],
                    "label": "VAT"
                }
            ] for item in data["cart"]["lineItems"]
        },
        "cartPriceTaxes": [
            {
                "tax": sum(item["price"]["totalPrice"] * 0.25 for item in data["cart"]["lineItems"]),
                "taxRate": 25.0,
                "price": sum(item["price"]["totalPrice"] for item in data["cart"]["lineItems"]),
                "label": "Total VAT"
            }
        ],
        "deliveryTaxes": {
            item["identifier"]: {"tax": 25, "taxRate": 25.0, "price": 25}
            for  item in data["cart"]["deliveries"][0]["positions"]
        }
    }

Response is also signed using the response body and shop secret.