Alistu,
I have worked a lot with the OptiSystem 13 OFDM components and also the Matlab component and I have not seen this bug before. If I understand correctly you have implemented your own OFDM Modulation in Matlab and OptiSystem Constellation visualizer is giving the strange sub-QAM constellation as well. I would need your project file to determine whether this is indeed a bug, but my feeling is that it is not in fact a bug.
Regards