The Timesheet portlet in the Classic Interface does not show timesheets which have been submitted and approved and posted with zero hours.
Release : 16.0.2, 16.0.1, 16.0.0, 15.x
If the timesheet has no tasks associated to it, it won't display in the Classic UI. It will still show in the MUX.
A task can be associated to the Timesheet, and assigned 0 hours for each day.