AA vs 38 is a 86.7% to 13.3% race preflop. So, you would expect AA to lose 13 times out of every 100 which I wouldn't call a bad beat. Its poker, if the best hand preflop always held why even have the flop/turn/river? With that being said I can't imagine what would possess a person to go all in preflop with 38 unless they were super short stacked in a tournament or playing against a short stack.