Pretty simple: flagging an answer while using the First Posts review page doesn't enable the "I'm Done" button. I'm under the impression that flagging a first post would be the type of action encouraged in the review process and consequentially I conclude this is not intentional and is therefore an error.
Okay, it seems that this issue happens only when the post is immediately deleted (denoted by the "this post has been deleted" message). I do not have this issue when the post is flagged but not immediately deleted.
Still, this sounds like a bug.