A journey on wings
A-36A was never officially called ANYthing but "Mustang." It *DID* have an unofficial nickname of "Invader," coined by the pilots and groundcrewmen of the 27th Fighter-Bomber Group in the Italian Campaign.
Both NAA and USAAF...it was always "Mustang," and the "Apache" misnomer came up as late as the 1980s when folks who didn't really know the score saw old NAA advertisements in magazines and on posters for the "Apache" (the NA-91 that was known as Mustang Mk IA in RAF; P-51/P-51-1-NA/P-51-2-NA before all but a couple became F-6A photo-recon Mustangs) and misidentified them as A-36As.
If you need documented proof, get back to me and I'll gladly send you scans of the USAAF and NAA documentation.