Signatures & 'Promotion and Demotion' feature

Document Property Promotion and Demotion is a Feature in versions of SharePoint designed to synchronize specific metadata across documents and its parent List columns and/or fields.

Property promotion refers to the process of extracting values from properties of a document and writing those values to corresponding columns on the list or document library where the document is stored.

Property demotion is the same process in reverse. Values are read from list columns and written to document properties.

The fields that are synchronized are the Title and Author field. Any changes to those fields when the 'Promotion and Demotion' feature is active will change the content of the document, consequently invalidating the signature. By disabling 'Promotion and Demotion', this synchronization does not take place and the signature will be invalidated only in the case where changes occur directly on the content of the document.

You can learn more about Promotion and Demotion here.