My paint is dark and blotchy in places and is flaking off with a white residue/powder visible under the paint – what is happening?

It is likely that 1. There is moisture coming out of the wall; either it is still drying out from renovation work, or, it is still being fed by an external moisture source, and 2. This movement of moisture has activated salts in the walls. These salts are likely to have sat undisturbed in the walls since construction and the movement of moisture through the wall has resulted in salts migrating towards the inside of the room. This will often manifest itself as a white chalky powder in mild cases (see image below) and as very obvious fluffy efflorescence in other cases. Salts will take off all paints including vapour permeable paints as they migrate out of the walls. Often this flaking happens rapidly and readily. Salts should never be washed off they should just by dry brushed and vacuumed. If the moisture source has been rectified and the wall dries out the salts will eventually stop coming through but this may take months, even years. If the wall is drying out a silane primer can be used before painting (make sure the room is well-ventilated and masks are worn). Use this sparingly as it will reduce the vapour permeability of the wall. If moisture continues to come through the wall salts will eventually come through silane primers too.

https://www.lime.org.uk/silane-based-primer-beeck.html