Paint on external walls is flaking after mist coats and prep

The fact that it only appears on the external wall, and only around the edges/corners tells me that this is 100% moisture in your wall unfortunately. You had the work done in the summertime which means plaster and mist-coats are fully dry by this point so we can rule that out. Now its wintertime and weather is wetter you have the issue appearing, which leads me again to believe its water in the wall. I see this all the time. It's doubtful that it's anything to do with caulk per say as others have said or you would notice it all over. You would not have noticed it on the mist-coat though as standard emulsion is breathable so would allow the moisture to escape through it. You noticed it on the top coat as I'm willing to bet you have used a vinyl paint eg valspar v500 or v700 blend which creates a waterproof barrier. This means that moisture will push the paint off the wall in order to escape as the room warms up. You can try Zinsser peel-stop as someone mentioned which is a binder/sealer for peeling paint, but this will only work if the moisture causing the flaking paint has been dealt with and fully dried out or it will keep happening. Sand away all the flaky paint, put 2 or 3 coats of peel stop on the areas, allowing it to fully dry between coats, then apply top coat. If this doesn't fix it, then you will need to get a damp company to survey the wall and see what the issue is.

Hi Did you use a flexible filler caulking around the tops of the ceiling and tops of skirtings all the corners it can react and peel on those parts

Moisture

While water and moisture are the most common causes of peeling, the opposite can also be true - excessive dryness can lead to peeling paint.

This is General Moisture but to fix the problem you can use a paint called peel stop primer by Zinsser but sand and clean the areas first.

If it’s not on the main walls it sounds like the caulking, sometimes cheaper ones cause the paint to flake off it’s like a hard brownish crust, check the tube and make sure about over painting. I’d rub the flaky down well, oil base prime or seal and go over it. I use unibond water based no nails.