Boards give way at the bottom edge and at the ends first, which is exactly where nobody is looking. Paint needs something sound to grip, and a failed board is not sound: it keeps moving, keeps taking up water and keeps breaking down underneath whatever is put on top of it.
The new paint then lifts along that one board while the rest of the wall still looks perfectly good, which is the giveaway that the surface was the problem all along. The job looked finished and lasted a season or two.
That is why repairing an exterior and replacing weatherboards where they need it comes first on our jobs, rather than being work you have to organise separately. Which boards can be saved and which have to come out is part of working out the job, and a replacement is made good so the wall reads as one surface again instead of as a patch.