Why did Vault not send out an Expiration notification when the document is expired?
Check that the document is already in the Destination State. For example, if the Expiration Start States and Expiration Destination State are the same, the document's state never changes.
When Vault sees this, it assumes the document is already expired. This is because it is in a Destination State for when the document becomes expired.
The workaround for this is to change the state of the document, at some point, during the document's lifecycle through a workflow or a user action.
Vault Help Documentation: Configuring Expiration Policy in Vault