It can be noticed that not all the records that are shown in the Modern UX's Timesheets tab (under the Timesheets module), are present in the Prtimesheet table. Where does this tab get its information from?
Prtimesheet table is populated with a record, for a particular time period, and a particular resource, if that resource touches the Timesheet under consideration. It does not hold records of all open timesheets.
There is a logic implemented in the Timesheet tab to consider this, and show open timesheets based on whether a time period is open or not, amongst other factors.