When the mouse is hovering over a Document Notification, an error occurs: Object not Found.
The hovercard is looking for a version of the document that no longer exists.
This is currently working as expected. This function looks specifically for the version that is created with the notification. If that version is deleted, or if it was up-versioned in a workflow, the hovercard information does not appear in that notification. Click the link to navigate to the document.
Vault Help Documentation: Notifications