Brand New 5D2 banding - return?

[LostArk]

I received my new 5D2 from B&H last week, which I bought specifically for low light use. However, I've noticed a strange banding pattern emanating from blown highlights against a plain background (examples attached, ISO 6400). It doesn't look like regular banding or sensor bloom, so I'm not exactly sure what to call it. Dark horizontal streaking, as it only occurs horizontally? I've never experienced this phenomena on my 7D (or any other camera I've owned), even at ISO 12800. Is this phenomena normal or should I send my camera back? Thanks for the advice!

 

[Mt Spokane Photography]

You can cause a camera to do strange things with bad exposures. I've never seen the black band, but have no idea as to your processing. It could indeed be a issue if it occurs in a properly exposed image.
Here is a low light image from my 5D MK II at ISO 6400. You will see some light areas in there, but they are just light colored stage props in the background.

 

[LostArk]

You can cause a camera to do strange things with bad exposures. I've never seen the black band, but have no idea as to your processing. It could indeed be a issue if it occurs in a properly exposed image

The samples I posted were 50% crops from out of camera RAW files, no processing. The images they are from were properly exposed.

This one was ISO 5000, f/2, 1/60 sec, no processing other than crop / resize: