Newly rendered walls showing hairline cracks

Certainly in my book it is. As scratch coats would never have been developed in the first instance the whole point is that it is a tighter coat and a key for the final application 2nd coat to adhere and before each application I recommend either uni bond pva or sbr to seal both coats and even added to the mix both coats , I have been in the construction industry all my life as a bricklayer from 16 years old now 60 I can say I know that’s right , how ever different backgrounds have have different effects on suction but shrinking is obvious without the prep and a scratch coat Hope that satisfies as an answer regards Ian lamont

The problem by the sounds of this is he probably just used normal building Sand and no lime in the mix . Charing Sand and lime in the mix . Render will normally crack as movement but with the lime it acts like a self healing process when rain hits it