I think that is the bug, and in the other post I have referenced, this issue has been stated that it acts differently from what it supposed to do, literally skips the sorted argument in GPU implementation.
In CPU mode, order is not correct.
Also, in the source code, there are two types of implementation for unique in both CPU and GPU which acts differently w.r.t. value of ndim but currently it has not been fixed and the issue on github page is still open.