During backups Duplicati only uses the metadata (such as modified or last-accessed date) as reported by the operating system (which in your case appears to be Windows) so it sounds like something else in your OS might have “touched” the file.
Did you notice this timestamp changed: True log for any other files? If it were a daylight saving related issue I would expect it to appear for ALL files on the first backup to run after the system clock was changed…