When you say the walls were painted a few months after being plastered (rendered) I think the problem may be that that the render had not dried out fully in depth and this can sometimes cause problems such as you have described.
I'm not familiar with the term 'mapping' but you often see what you describe happen on new build housing where the painting has been completed more or less as soon as the walls have been newly rendered.
Since you say the coating seems OK otherwise I wouldn't worry about it. People sometimes suggest the need of a primer because it obviously increases the price of the job.
I wouldn't recommend Farrow & Ball for exterior use though since their formulations are weighted towards giving a rich aesthetic effect rather than long term performance.
Sandtex or Dulux Weathershield will perform better, even if you have to compromise slightly on your preferred colour since the matching can never be 100% accurate due to the different formulations.