Since, as I found after posting, GoogleDrive actually does not update the time of the response spreadsheet of a Google Form when a new form is sent in, there is no error in rclone, and no possible solution for my problem.
An obvious workaround is to 'copy' instead of 'sync', or to just 'touch' the files you want to force syncing, if there are just a few.
I need to keep only one file up to date, so I inserted a 'rclone touch <that-form-response-file.xlsx>' before the sync job.
I think this issue is solved.
Giorgio