If the 1st level approver has the permission 'Manager Entry - Override Holiday Approval Workflow permission', once they approve a leave request this will not go to the next level approvers and will instead be directly authorised and ready to be sent to Payroll.
To check if the 1st level approver has this permission:
Go to the employee's profile and click Security, then Permissions.
Click Edit Employee Permissions and search for Manager Entry - Override Holiday Approval Workflow.
If this is enabled, disable it and click Save.
It's also possible the permission is not assigned directly to them and is instead inherited from a role or post.
Go to the employee's profile and click Security, then Permissions.
Scroll down this page to look at all the permissions inherited from roles.
Under each 'Inherited from role' section, search for the permission Manager Entry - Override Holiday Approval Workflow using the quick search bar.
Once you have found the role that contains the permission, you can either disable the role for the employee or remove the permission from the role. Please note that removing the permission from the role will automatically remove it for all employees with this role.
To remove the permission from the role:
Go to Setup and click Roles & Permissions.
Search for the role and click it.
Click Role Permissions and then Edit Permissions.
Search for Manager Entry - Override Holiday Approval Workflow and disable it.
Click Save.