When a user updates the value for the Label field of a Vault workflow, the Document Timeline View still shows the previous value for this workflow.
This is working as designed.
The Document Timeline View gets data from the Workflow History Table and shows the label of the workflow at the time that the workflow is completed on the document.
The updated workflow label is reflected in the Document Timeline View on any document that goes through the affected workflow after this change is made.
Vault User Documentation: Using Document Timeline View