I flagged and voted to delete a low quality answer
After a while, the flag was deemed helpful, but the answer isn't deleted by moderator. Turns out the author deleted the answer which cleared the flag and delete vote status automatically, and then he un-deleted the answer. Now I can vote to delete again, but I can't flag it anymore because I already flagged it before.
Does this need to be fixed or is it by design?
EDIT: the answer is modified after I asked this question, now its quality is fine, but my question remains.