Why are purchase requisition approvals going to previous manager (a user who has now left)?

Summary:

Purchase requisition approvals going to users who have left the organisation, although reporting lines and cost center managers (which is what the approval rules use) have been updated.

Content (required):

We have approval rules set up for purchase requisitions which use requester, then up the line management hierarchy until it reaches someone with a sufficient job level. And then it goes for approval to the cost center manager (if they're not already included in the first approval flow). We have two examples of users who have left appearing in the purchase requisition approval flow, although reporting lines and cost center managers have been updated.