The normal thing to do before a ceiling is artexed over existing plaster is to apply a bonding agent, PVA glue watered down for example. It seems that the water based emulsion that is being applied is causing the artex to loosen. The only thing to try is to apply PVA over the top of the artex and around the area that it has come off and try to patch and paint again after it has dried. Whenever I have had to fill a hole in a ceiling that has been artexed I have used polyfiller and tried to copy the pattern of the artex, this is always on a very small area and works well but may not over a larger area.