PO approved through email but not update approvers worklist

We are using EBS version 12.2.4

We have PO for approver to approve. They can either approve through the worklist when the approver login to the instance, or the approver can approve through email.

We have one situation that the approver has already approved a particular PO through email, but in the approver's worklist, it is still showing the PO is needing that particular approver's approval. But if you go to the PO summary, that PO has already been forwarded to the next level of approvers.

Please advise how I can begin to troubleshoot this issue. Thanks.