When you comment on a document card using Buzz, that comment is tied to the version that is currently active in the Details view. By default, the active version is the most recently uploaded version. However, if you select a non-current version in the Versions panel in the Details view, that version temporarily becomes the active version (until you leave the Details view). Any comments you make at this point are tied to that non-current version.
By default, when you open the Details view for a document card, all comments for the document are shown regardless of version. This may make it difficult for users to understand the story behind the document, especially if comments are spread over various iterations of that document. However, you can get around this problem by filtering comments by version.
Filters are applied automatically when you select any version thumbnail. For example, if you select "version 2" of a document, only comments for "version 2" are shown in Buzz. If you want, you can choose to show all comments for a document regardless of the selected version.
To change the filter settings for a document,
Click the document card to open its Details view.
Select the desired filter in the dropdown next to the document name in Buzz.