So just to post an update on this issue since it was solved through support. Previously when you wanted an app image to be the background image for an XY column, you needed to construct the URL to https://www.appsheet.com/template/gettablefileurl with your app name, table name, file name, etc.
There has been a recent change to start constructing full, signed URLs for image and file column values for security before they are passed back to the client. This means that for XY columns you no longer need to construct the full URL for the background image. Instead, you can now just reference or de-reference to the image column. This is what fixed the issue in this case.