Bright highlights clipping to black / blue in video mode?

[kev8d]

Yesterday I was filming a wedding with a new video light (1000watt lowel DP light), and I noticed that the brightest part of the light appeared black with bits of blue in my footage rather than white. (See sample jpeg)

Can anyone explain this? I was shooting with a 5D mkII.

 

[kev8d]

Tonight I experimented with the light and several cameras. The clipped highlights appears to be due to high ISO settings on the 5D mkII. After around ISO 1000 the clipping appears.

I haven't noticed it at similar ISOs on our 7D and 5D mkIII.