I may be wrong of course but it does not look like posterization and certainly does not look like just chroma noise.
Also it is ALWAYS in parallel to the longest side no matter what I shoot. Since I do not shoot video I translate filming to shooting.
I would rather not increase ISO since it lowers Dynamic range. Experiments made with high iso and trying to recover shadows while keeping highlights from burning gave poor results. I have to keep exposure lower to save highlights (less DR) so shadows are even worse (and there was banding).
ISO 50 behaved better (although it is a digital iso, the way camera handles it saves us from quite a processing). EDIt: It betters 5D3 in shadow lifting too that's why I tried it with 5D4.
To tell the truth some shadows have to be kept as shadows to keep a photo natural so this can be controllable in some cases. But in really HDR demanding scenes (like an open window for example) the solution remains 5D3 + ML (or the classical bracketting and combining

)