Why didnt Sasuke notice that Killer B had used a substitution?
The tentacle Killer Bee substituted with was literally his body, had his chakra nature, was made of pure chakra (as all tailed beasts are) and probably had more raw chakra in it than most shinobi despite just being a small piece of a tentacle. Sasuke believed he had just lit Killer Bee up with Amaterasu, so the only reason he would have to suspect that the tentacle was not Killer Bee, would be if he checked chakra levels- But since he had just lit the tentacle up, it would be reasonable to believe that Killer Bee was low on chakra. It would have taken someone with the prowess and intelligence and experience of Itachi to realize that it was not actually Killer Bee, and Sasuke was just not on that level regardless of how tired he was. Even Karin would not have known the difference for the same reasons Sasuke would not have even if he studied the tentacle in depth while it was transformed. The only way to know would be to guess and dispel the substitution jutsu on it on a whim which is something that would have come from experience they both lacked. Only someone who had fought a tailed beast in earnest to the death and known their full capacity would have realized that Killer Bee, the only perfect jinchuriki at the time, went down way too easily and decided something was up. Basically- It was Sasukes Arrogance of youth and over confidence in his own abilities that allowed Killer Bee to escape.