The 5e MM also states: "For a rakshasa, death on the Material Plane means an agonizing and torturous return to the Nine Hells, where its essence remains trapped until its body reforms- a process that cart take months or years. When the rakshasa is reborn, it has all the memories and knowledge of its former life, and it seeks retribution against the one who slew it." Note that it doesn't state exactly which body reforms. Perhaps the rakshasa reforms in its original devil body... which could be a pretty amazing story hook for a campaign. The party takes out a CR 13 rakshasa... and then some time later, they get ambushed by a
very angry CR 20 pit fiend, who seems to be nursing a grudge...
Alternatively, maybe the diabolic spirit reforms in the Nine Hells in its new rakshasa body. In this case, it's probably a figure of some derision and amusement for the other devils. Think about it... you've just shed your diabolic body to create a new rakshasa form on the Material Plane. Your existence back in the Nine Hells in rakshasa form implies that somebody killed you, and you've recently regenerated. It's like wearing a big sign saying: "Some humans killed me!".