Why is new paint flaking off one area?
I am not sure what a mist coat is but it looks like there was dust that kept the mud or paint from sticking. If you had used a skim coat of mud without cleaning the surface as it dries it can flake off. The same is true on top of the mud--if it's dusty the paint may stick to the dust and flake off when dry, so in both cases a good wipe down is needed to prevent these kind of problems.