Why does permanganate oxidation of 2-nitronaphthalene not form phthalic acid?
Looking at the substituents; NH2- activates the ring, making it more reactive than dibenzene ring system (easily oxidised), and the NO2- substituent is very deactivating - (strongly electron withdrawing), making it less reactive (not easily oxidised).

Although  a powerful oxidising, optimised oxidising agent (KMnO4) is being used, it is only able to oxidise the 2-naphthylamine compound to produce phthalic acid.

and 2-nitronaphthalene is very unlikely or at least takes longer to completion with very little yield.