Summary

As soon as a user is added to an issue as an approver, that user is treated as a customer for the purposes of notifications, even if they're an agent. Unless they then remove themselves after approving a given issue, they'll miss out on receiving any subsequent internal notifications.

Suggestion

If a user is added as an approver to an issue, if they're an agent, keep treating them as an agent for the purposes of notifications.

The use case here is when a customer is set as an approver and merely replies via email saying "This is approved" but doesn't visit the portal to actually click the Approve button. The agent can add herself to the Approvers field and manually approve it to transition the issue through the workflow.