Fix? Move to 4K BluRay content. It won't stop bad content but pixelation like you noted is sadly part of the compression game streaming services must employ. So the only way out is to move to less compressed content like the new 4K players and hard media.
I have a samsung js7000 4K tv. Whenever watching dark scenes through cable or Netflix the blacks/greys always appear super pixelated and blotchy. Bright scenes are perfectly fine.
Has anyone else noticed this issue? I have my picture set to 'dynamic' otherwise unchanged.