Emails have HTML content in the subject line.  For example,

Note the text of <span style='white-space:nowrap'>System (No resources found to assign to)</span> in the subject line.

Steps to Reproduce

1. Open the OOTB Supervisor Workflow. 
2. Click on Approval Completed node.
3. Enable the node. You can see the subject line uses OOTB variable ${PreviousNode_completedBy}. 

4. The ${PreviousNode_completedBy_meu_fullName} variable is used in the body of the email. 

5. Raise an Add Request for a user whose supervisor is Terminated or has no supervisor. You can see for the raised request the Supervisor approval is completed by the system.

6. Check the email logs. You will see the HTML content in the subject. But the body shows correct data with HTML content.

Out of the box workflow variables ${PreviousNode_completedBy} and ${PreviousNode_completedBy_meu_fullName}  are sending HTML content in the Subject line of emails when an approval is completed by the system.