Yes, I wonder too if the heat treat of the stud was affected by the welding. it doesn't look like the plate or the weld failed it appears to be the stud.
The other odd thing is why were the other two studs welded and then the plate was zinc plated but the stud without the head was welded on after the plate was plated? Seems to me that you would install and weld all the studs then send the whole thing off to be plated afterwards.
Sorry you've had such bad luck. I checked my studs tonight (and two track days weekend before last) and all nice and tight still. Hope you're able to find a suitable replacement.