I guess put even more simply, the Sync to SFDC flow step does not actually run for leads that are already synced to SFDC. Once a lead is synced with SFDC for the first time any updates to the record flow automatically to the record in SFDC, you do not need Sync To SFDC unless the lead has never been synced.
If you want to change the lead owner in SFDC, you would use the "Change Lead Owner" flow step instead.