Yes, brussel sprouts do smell when microwaved. Like some other cruciferous vegetables, they contain hydrogen sulfide gas that releases during cooking. While it’s not harmful, it does stink a bit like Sulfur. The longer they cook, the worse the smell will be. By splitting them and reducing your cook time, you’ll significantly reduce the odor produced. That’s not to say they won’t still stink a bit, but it’ll be far more tolerable.