flask upload python-base64 image

I’m trying to upload a base64 image (encoded in Python) to my webapp using Flask.
I get the encoded image but get an error when modifying the src of the image element.

Ajax:

$.ajax({
    dataType: "json",
    type: 'POST',
    url: getWebAppBackendUrl("return_cam"),
    data: {
        image_bytes: img_b64
    },
    success: function (filter_bytes) {
        let data = filter_bytes.heatmap
        document.getElementById("cam").src = "data:image/png;base64," + data;
    }
});

server side:

@app.route('/return_cam', methods=['GET', 'POST'])
def return_cam():
    
    img_b64 = request.form.get("image_bytes") #get POST data
    api_result = client.run_function("returnCAM", image=img_b64)
    response = api_result.get("response")
    result_dict = json.loads(response)
    return json.dumps(result_dict)

The ‘data’ variable looks like this when printed in the console:

b'ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj/ADj...'

and I get ERR_INVALID_URL.

I guess there is some processing to do on this data variable…
Thanks for the help!

    img_b64 = request.form.get("image_bytes")
    img_b64 = img_b64.decode('utf-8')
    #...