Comments are added to a document prior to the initial Draft upload. A user downloads and uploads comments using the Offline review capability, Export Annotations. Any comments which are placed on the original file and are not added through the Vault interface appear as owned by the first reviewer to use Offline review.
This is a known limitation in Vault.
Vault has no way to distinguish Adobe Acrobat annotations created before or after export. It treats all annotations as new on import. It converts all of these annotations to Vault annotations (i.e. deletes the Adobe Acrobat annotation and replaces it with a Vault annotation).
However, the pre-existing Adobe Acrobat annotations already exist in the PDF rendition at import since Vault flattens them into the rendition on the initial upload and rendering. As such, they essentially exist twice after import: once flattened into rendition and a second time upon conversion with import.
The recommended practice is to not create any Adobe Annotations before uploading to Vault if the user intends to export and import annotations on that document.
Vault Export/Import Annotations Documentation: Using Offline Annotations