Sales Order workflow notification going to undesired recipient

I had set up the SO workflow notification to send notifications when the workflow state changes. This was working perfectly well (see snapshot below) with the right people getting the right notification

Recently one of our Sales Users created a customer contact (email id and other information). Since then a notification is also going out to this customer contact also when the workflow state changes. I checked the customer contact is not an ERPNext user.

Any ideas why this could be happening?