Partially signed document in signed-pdf-url

We send a document via API and save a signed version of the document when it ready.
We save signed version from signed-pdf-url field which we receive from callback XML. 

But sometimes we get a partially signed document without the certificate page. After some time if we requested the document again, it already contains the correct version of the signed document.

It seems that the signed version of the document is in progress when we receive a callback about signed.