FineUploader 5.14.0 - setParams not working with large files

Question

I've had the following code working for quite a while now (over a year) but a user has tried to upload a 14MB file and the extra data I post along with the upload seems to not get posted anymore.

In Chrome dev tools I look at the header of the (single) XHR and I see the data in the "Form data" section but nothing get's to the server which I don't understand.

Files that are a few MB or smaller work without issue. I've not found a magic MB limit yet.

The extra data is in the onUpload call back. board_hash is in the head of page.

var fu_instance = new qq.FineUploader(
{
    element: $uploader[0],
    template: 'agenda_file_template',
    debug: true,
    request: {
        endpoint: '/m/upload',
        forceMultipart: false,
        customHeaders: {
            Accept: 'application/json'
        }
    },
    autoUpload: false,
    messages:
    {
        noFilesError: "There is no files to upload. Select or drag and drop some files to upload.",
    },
    failedUploadTextDisplay:
    {
        mode: 'custom',
        responseProperty: 'error'
    },
    callbacks:
    {
        onSubmit: function(id, filename)
        {
            // File added to upload
            $uploader.addClass('hide-drop-msg');

            $btn_submit_upload.html('Upload').show();

            unsaved = true;
        },
        onUpload: function(id, name)
        {
            fu_instance.setParams({'board_hash': board_hash, 'parent': $parent.val()});
        },
        onCancel: function(id, name)
        {
            // Actually onBeforeCancel
            if ($uploader.find('ul.qq-upload-list li').length == 1)
            {
                // There is currently 1 & it's about to be axed
                $uploader.removeClass('hide-drop-msg');
                $btn_reset_uploads.hide();
                $btn_submit_upload.html('Upload').show();
                unsaved = false;
            }
        },
        onError: function(id, name, reason, resp)
        {
            // Specific file error
            if (resp.hasOwnProperty('auth_expired'))
            {
                window.location.href = auth_url;
            }
        },
        onComplete: function(id, name, resp)
        {
            if (resp.success)
            {
                var $parent_el = $('#'+$parent.val());

                $parent_el.find('.files').append(resp.html);
                $parent_el.find('.no-agenda-files').hide();
            }
        },
        onAllComplete: function(succeeded, failed)
        {
            // Every file is done
            $btn_submit_upload.hide();
            $btn_reset_uploads.show();
            unsaved = false;
        }
    }
});

My understanding is that chunking is off by default. Have I configured this wrong or am I in the wrong call back?