When you add a payslip note to an employee, you can't preview that note when viewing payslips while the payroll is in a calculated state. The payslip note only appears after the payroll has been finalised and sent to the employee, where they can see it in their online portal.
If you'd like to confirm the contents of a payslip note that will be applied for the period, hover over the yellow icon 📄 which shows to the right of the team members Payroll ID in the Current Period screen.