I'm not expert in dog, but i got cats with almost the same problem, a spot fur off in head and body. I sent them to vet and give some injection. After one week no more spot fur off. Then another thing, my cats also have some problem on fur loos every time we touch them fur will stick on our hand specially if its wet. Vet gave me fur spray eliminate this small micro minimo lice or fleas that is invisible to our naked eye. Up to now still a bit lossing fur i dont knwo why.
may be its normal, human also have falling hair every time we bath.