Why is a workflow appearing in a user's Active Workflows view after the user completes a task in the workflow?
The Active Workflow view lists workflows in which the user is a participant or the Workflow Owner. The user sees workflows on any documents where the user has the Owner or Coordinator role.
A workflow remains in this view for a user until it is completed. It is working as designed that a workflow remains in this view after the user completes a task if the workflow as a whole is still ongoing.
Vault Home Page Documentation: Active Workflows