Dragon Ball Super Goku Black vs Majin Buu?
It isn't really a Plot Hole. Goku and Vegeta grew absurdly strong in Dragon Ball Super in comparison to Dragon Ball Z. In the Planet Potafu arc which was before the Goku Black Arc, we see Base Vegeta completely toy around with Super Saiyan 3 Gotenks.Super Saiyan 3 Gotenks was comparable to a Super Buu who is the strongest version of Buu(Disregarding the fusions). The current Majin Buu is the weakest iteration of Buu and clearly doesn't have the strength the hold his own against a Base Goku and Vegeta, let alone a Super Saiyan Blue tier character.With regard to his regenerative abilities, Buu can regenerate. However, he can be destroyed if he's hit by an attack of intense magnitude. The Spirit Bomb for example, was strong enough to defeat Kid Buu. Super Saiyan Rose Goku Black is more than strong enough to easily destroy Buu if he desired.Finally, With regard to being absorbed, A much stronger version of Buu (Buuhan), struggled to absorb Vegito and only managed to do the same when Vegito let him to. Goku Black is much stronger than the Vegito in the Buu Saga and Majin Buu is much weaker than Buuhan. So even that wouldn't have been possible.