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My hunch is that it is related to the lossy compression. If you throw a couple of bits of information away, there are bound to be places where you can see that, and very gradually changing sky-colours would fit that scenario.
I would think that using only 11 bits in a non-linear encoding would harm smooth gradients, but it doesn't. I have examples of sky where I have processed them to extreme, really extreme contrast levels, and there is still no sign of loss in tonal resolution.
In the preview window in CaptureOne, posterization is always ugly, but it is not there in the processed file, even directly to a small size jpg.