I agree. Simply stating "this is a known bug, here's the bug report" is a pretty crappy answer. If that's all the answer that can be given, it would be better to just close the question as a bug report. That's one of the boilerplate close reasons available on Ask Ubuntu:

[![close reason for bugs][1]][1]


If, on the other hand, there is useful information in the bug report. Then, instead of just linking, copy the information into your answer [as the OP of your example did][2]. 

So yes, in my opinion, an answer that's just a link to a bug report is useless. It is not an answer [as defined by SE][3], and it would be better to post it as a comment and close the question. 


  [1]: https://i.sstatic.net/VVPJW.png
  [2]: https://askubuntu.com/a/763110/85695
  [3]: https://meta.stackexchange.com/q/225370/203101