Hi Everett,
Are any of the filters used within that Quick Base to Quick Base Sync set up based on a formula field that may not always be true? It could be a formula field not always being true or not evaluating in the way you expect if it is part of a filter or key. Do you receive any error messages for that Sunday Sync or does it show up in the Sync history as a successful sync? I am curious if the same happens with your manual sync or not.
Something like this sounds fairly unusual and usually a good time to reach out to Care to see if they can look for any history on this Sync and what might be causing it to become de-synced if you aren't receiving error messages. You can contact our support when signed into Quick Base by clicking on the question mark icon in the upper right hand corner and selecting Manage Support Cases or directly
here.