I made some post-processing and came to the point to see the gradient in the sky turns into zones separated by sharp borders, which I believe to be classic example of posterisation.
Firstly I thought that will be resolved easily by local reduction of sharpness. However, that only somewhat blurred separation borders, the zones retained.
Than I went to removing all things I did, one at the time, but whatever I do, I cannot restore the gradient.
What should be the first to look for in these circumstances?