nano ~/custom/timesheet/core/modules/pdf/pdf_rat.modules.php :
$conf->global->TIMESHEET_INVOICE_TIMETYPE = "hours";
It looks like no parameter set here https://dolibarr/custom/timesheet/admin/timesheetsetup.php is taken into account in the PDF output here https://dolibarr/custom/timesheet/TimesheetReportProject.php?action=reportproject