Behavior
When deploying a new version of a workflow, after updating the email or emails, the current running in-flight workflows (on an older deployed version) are suddenly using the newest version of the task email. If you modify the email on a task, as soon as you deploy it, workflows still running on the older version will immediately change to using the newest version of the email (if that step has not yet been reached on the older running workflow version).
Explanation
This is expected behavior. All versions of the workflow will use the most recent version of the notification email. The task notification email is saved to the database when deployed and use the Event name when associating the email to a running instance. When a new version of the workflow is deployed, it looks for the name of the Event and updates the Task notification email. This is the reason why when deploying a new version of the workflow that has a different email content for the task notification, it will update the email content for running instances as well.