That's the trouble with relying on synchronised based solution as a backup, you fall into "false" sense of security and not monitoring it's completion on regular basis. It's quite possible you haven't synced for the last 6 days. Microsoft doesn't backup anything for you. You could try checking for "deleted" items in case it was the case with older copy overwriting and deleting newer files. I am sure you may have already searched for it, but try https://support.microsoft.com/en-us/office/restore-deleted-files-or-folders-in-onedrive-949ada80-0026-4db3-a953-c99083e6a84f (not sure if link will go through)