Cause
The illustration is the result of the finishing coat of plaster being over polished leaving little or no porosity for the waterbased coating to key onto. It can be seen that there is no latent plaster adhering to the reverse side of the paint flake.
This can also occur when the first coat of a heavy pigmented paint is applied directly onto the surface without thinning to act as a sealer coat.
Remedy
Scrape back all loose material to a firm edge – dampen the exposed plaster with clean water and apply a fine plaster filler to the surrounding area. When dry, lightly rub down, dust off and apply a sealer coat of the waterbased paint followed by the required finishing coats.
Note : for face filling, fine ready mixed plaster filler is suitable.