It can be turned from round as you say, but ideally should still have 2 flats on opposing side to prevent it from spinning inside the T-slot while tightening.
Also, pay attention to this corner fillet on the nut (red arrow). Looks like there isn't much of a chamfer on the casting corner so this condition could put the full force on the corner contact vs the top face of the nut resulting it a cracked T-slot. You can either turn a sharper internal corner, or recess it a bit, or loosen up on the tolerances... anything that mitigates this. Also I would keep the top face of the nut say 0.020" beneath the top face of the T casting just so it has a safe distance. If its flush or bends over time it will bottom out prematurely & not be fully tightened.