You mention the "dissimilar materials" thing OG. I've been giving it some more thought. I really don't think it's that at all.
That happens when substrates such as timber/steel beams/masonry are right next to each other and plastered over together.
They expand/contract with different rates of expansion. Thus, the plaster is put under tension.......... and cracks.
You don't have this here. Also, in a previous post, you mention cracking in several different places.
I would guess that it's simply plaster shrinkage cracking. Usually occurs if plastered on a hot day and it dries out too quickly.
Another favourite is when the substrate (concrete blocks here) is far too dry, and it sucks all the water out of the plaster.
I guess this cracking has been in place since day 1.
As for exterior... same thing can apply. Repeat filling will eventually rid you of the problem.
Surveyors often know little about plastering, so they tend to look for "mechanical" problems.