I just had a similar experience accessing Updraft backup files in S3. I thought the last backup had ended on Jan 9, but it turned out to be an issue with the S3 filtering system… if you sort the backup files in your bucket by date, it will only sort the files that are visible on the page currently (300 in our case)… but you may need to click to the 2nd page (ie. results 300-600) or further and sort each page by date to find the recent ones.
Hope your issue is as easily resolved as mine was….