Okay serious headcanon,
Kaiba spent a few days with Atem on the other side. He got to know the spirits of those who’d passed and learned more of his past self. Though it is some disconnect from the version of who he is now.
Kaiba wonders just why his soul was sent back to the wheel of reincarnation verses everyone else who got to stay on the other side and wait for Atem. Though due to ego and putting himself as someone invaluable he rationalizes it as his soul isn’t done with it’s goals yet, and Atem needed him. That he still had a lot to prove from being Priest to being CEO. He was born to be the spear head.
Kaiba didn’t get to spend too long on the other side as his living body was locked in the Cyberlink device and to tear it open would rip his mind apart. So Mokuba was forced to wait on his brothers return. Three days in the after life, and while there he took readings and made attempts to perfect his devices for his brain to be on a higher plane. However, like a needle puncturing a balloon or bubble the worlds balance was fractured.