This is by design. An answer deleted by reviewers without the privilege to delete (i.e., below 20K) can be undeleted by the post owner with one click.
However, doing so raises an automatic moderator flag "disputed review". The moderator then makes the call on whether the answer stays or goes.
Source: Answers deleted via Recommend Deletion votes in review should not be undeletable by the answerer
This may be discouraging at first: it takes six recommendations to delete, all of which get overruled with one click by the author. But in reality, such undeletions are rare.
To confirm the above, see the query Undeleted answers that were deleted from review. It returns only 15 answers.
I also wrote a query Negatively scored answers that have been recently undeleted. The word recently is redundant: there are only 127 such answers in total. (This query does not distinguish the causes of deletion and undeletion.)