Based on the reported outcome, this points to a bad copy rather than normal behavior. After exchanging the lens, the replacement still showed some ghosting, but it was much less pronounced and the lens was usable at f/3.5. That suggests some flare/ghosting can be normal with an ultra-wide fisheye, especially around bright light sources, but a strongly shifted, ugly asymmetric flare at maximum aperture is not typical and can indicate a defect or optical misalignment. If stopping down to f/5.6 is the only way to make the lens acceptable, that is a strong reason to return or exchange it. In short: mild ghosting may be normal, but the severity you described is consistent with a defective sample.