Delegate Approver Able to Approve Their Own Expense Report

Hi everyone,

A user was able to able to approve their own expense report when setup as a delegate for an approver. This occurred when setting up a vacation rule. For eg: User A submits and expense report. User B (Manager of User A) sets vacation rule where he delegates to User A. Now when User A submits an ER , it goes to approval to User A due to vacation rule set. We have enabled prohibit self approval, even then it is not working

An individual should not be able to approve their own expense report. Can someone please help how can we achieve this.