A change made to the person defined in the Coordinator lifecycle role is reversed in Vault.
The Coordinator role is not updated using the Replace Participant action when a document is in an active workflow.
When a document is in an active workflow, the Coordinator role must be changed to another user using the Replace Participant workflow action.
If this is done by updating the Sharing Settings, those changes revert to the user who is previously defined in the Coordinator role when a user task (example: Assess Outcome) is assigned to the Coordinator role during the active workflow.
Vault Lifecycle Documentation: How to Replace Workflow Participants