This is not a bug, as far as I know, it's the intended behaviour. Putting aside the programming complexity (low or high), I think UX-wise, it's the kind of control which serves its purpose better when it's always displayed, rather than sometimes hidden.
If the control is always here, its use is pretty clear, and even if irrelevant for just one answer, at least tells you you could order the results if there were several answers.
If it's hidden for 1 answer, then shown when there are several, then it's disturbing (furthermore with this current bug on sorting controls display). Be sure a good chunk of "bug reports" will come in, reporting the controls are "missing" when their presence is not expected to start with.
Plus, as VLAZ mentions in a comment, the control is applied site-wide, all the more reason to display it consistently.