This applies to the Premier and Master payroll edition.
To determine why an employee's leave accrual disappeared, the Leave Scheme Parameters screen will have to be reviewed.
If multiple leave schemes have been configured, verify if any of them are based on length of service or grade bands.
Navigate > Configuration > Human Resources > Leave Administration > Leave Scheme Parameters
A possible explanation may be that certain leave types were set up based on particular grades, and when the affected employee was assigned to a new grade, the current leave type may not have included the new grade, leading to the disappearance of the leave accrual from the Leave Balances screen.
Depending on the company's policy, the solution may be to add the employee to the leave scheme that includes the new grade.