That is a great question. I am not sure. Given my amazing attention to detail, I probably missed a super rare bird flying past.
In scrolling through the previous images, I can see it start and move left to right. Just appeared atop the lens in the image I selected.
Here is the previous shot...
So, I wouldn't call it flare.
BTW, these were with the 150-600S. Not the DO.
I'll try to not bokeh peak too much, but the 400 DO II may suffer from the same issue you had with the 500 Mk I. Overall, as I scroll through images, I am very happy with the 400 DO II. I am sure I will post others. But if I am spending $7000-$10,000, I am going to put these lenses through their paces.