Dylan from the Jira team pointed out that if you are the Author of a review where you are not the moderator it is confusing that you don't get a complete button. Specifically, Anton created a review of Dylan's commit, Dylan responded and then came to ask what he was supposed to do next.

IIRC we removed the author from being able to complete in the push to remove any unnecessary steps from the default workflow. Possibly we went too far in this instance.

This seems to have been broken in later releases. We have Author with permission to complete and uncomplete but the author never sees that option.