fixed aperture problem with exposure

I bought sigma 18-35 1.8g with fixes aperture at 1.8 When i shoot in aperture priority mode for exemple at f4 it makes one exposure and when i shoot in f2.8 i see there is some highlights blown out...hows that possible when its constant 1.8 aperture lens and also thts should not affect aperture changing cause camera just change shutter speed?????