I suspect that any banding probably comes from having to stretch a tonal range too far in post processing - usually when trying to compensate for the reduced dynamic range or compensate for a significant degree of under-exposure.
At high ISO settings exposure is far more critical in wide-dynamic range scenes ... if you under-expose by, say, 2 stops (not particularly noticeable to the human eye and easy to do) you've just thrown away a full 3/4 of the information that you could have captured, and when compensating in post-processing - for a high ISO shot where your "safety margins" are already vastly reduced - you'll often get banding.