I'm not claiming completeness of the list, but these are the reasons that come to mind right now:
A VLQ flag is marked helpful if
The flagged post is deleted.
The flagged post is edited.
The flagged question is closed.
A moderator clears the flag as helpful.
Deletion can be a deletion by users other than the author, or by the author themselves. If the author deletes a flagged post, the flag is marked helpful, and the review is marked "invalidated" in the timeline. If the author undeletes a post that was deleted from the review via "Recommend Deletion" votes (they can't single-handedly undelete if the deletion was by three delete votes from 20k users), a system flag is raised so that a moderator looks at the situation. If deletion and undeletion were both done by the author, no such flag is raised.
An edit to the post before the review was completed also shows the review as "invalidated" in the timeline. An edit from the review queue completes the review.
A closure of a VLQ-flagged question before the review is completed also "invalidates" the review, I think.
A moderator decision also "invalidates" the review. Often, a moderator clears a VLQ flag by deleting the post, but it also happens that the flag is marked helpful without deleting the post because the handling moderator thinks there is sufficient reason for the flag, but not quite enough reason to delete the post.