A viewable rendition downloaded from Vault does not have a manifested e-signature after going through a workflow that created one.
The source Document likely has a password or Security is enabled on it.
Vault cannot manifest an e-signature on a Document that has security on it. The security has to be removed and the workflow re-run in order to capture an e-signature. The Screenshot below is an example of a secured document.
Vault Help Documentation: N/A