In the Manga?
In the anime Majin Buu was always a transient being, that's why Supreme Kai went to the earth to find Gohan, because Kai had fought Buu and knew how strong he was so he was looking for more allies that were of equal or greater strength to himself to defeat Buu again.
As I google search it though people are talking about a retconn, but I'm not really clear on what was retconned. The anime was always Buu just existed and Bibbidi found him and woke him. I'm not entirely sure which way it goes. Or what's being retconned.
From what I've read it was just some mention that Toriyama said in a QnA. Even further DBS mentions has Majin Buu as an always existing being, Beerus is surprised he was defeated, and Frieza worrys about how strong he needs to become when he heard Goku beat Buu. King Cold told Frieza to never fight Buu.