When we use the a!fileUploadField, we're saving the document Id to a CDT var

You can not use the temporary document id generated before the form is submitted. You can search an find many forum posts related to this.

As soon as you upload a document, it's assigned a temporary id (and the document itself is kept in temporary documents folder). Once you submit the form, document is moved to target folder and a permanent id is assigned. Use the document id that is generated once you submit the form.

Hi Nick,
some more details to the correct answer of Akshaya: as explained in the documentation (scroll to the notes : forum.appian.com/.../SAIL_Components.html the Document object is updated with the final ID after the form is submitted but integer values that contain the ID are not updated even if they are in a CDT. From the upload form pass the Document variable to the process and not the id, you can add a Script Task with an output node that writes the Document variable in the field (Integer) of your CDT to store the final ID. This works with arrays of documents too.