In the past decade, the Dragon Ball franchise has again returned to prominence in both Japan and the West, and part of this renaissance was kicked off by productions such as the Resurrection "F" movie. This film brings back the warlord villain Frieza, who returns to fight a much more powerful Goku. Most concerning to Frieza is his opponent Goku's increased power level, which has allowed him to beat Majin Buu.
Majin Buu's defeat logically shouldn't be a surprise to Frieza, who witnessed Goku's battle with him from Hell. There's an explanation as to why Frieza's discovery of Buu's defeat makes sense, however, and it deals with the canonicity of the anime.
Frieza's Return In Resurrection "F"
The film's beginning has Sorbet and Frieza's other remaining forces using the Earth's Dragon Balls to bring the fiend back to life. Once he's firmly back in the land of the living, Frieza makes it clear that his only real goal is a rematch with the Saiyans that defeated him, namely Goku.
Sorbet and the others try to warn their master against doing so, knowing full well how much tougher Goku has gotten since he last faced Frieza. The reptilian warlord scoffs at their rebuffs of his plan, wanting to go full steam ahead with it. He's only stopped in his tracks once it's mentioned that Goku has defeated the feared Majin Buu, which shocks even Frieza. After all, Buu, along with the Gods of Destruction, was one of the only beings who Frieza had been warned to never cross.
This is an interesting way of showcasing how much higher the power scale has gotten in the series, but the scene was also particularly confusing to fans of the Dragon Ball Z anime. After all, Frieza had been viewing the final fight between Goku and Kid Buu when he was in Hell and thus should know how things turned out. There's a reason, however, as to why this scene was overlooked in Resurrection "F."
How Frieza Didn't Know About Buu's Defeat
The scenes where the denizens of Hell got a sneak peek at the battle between Buu and Goku was only in the Dragon Ball Z anime and not in the manga. This scene and the filler episodes of the anime aren't canon to the story of Resurrection "F" or logically to that of Dragon Ball Super. Thus, Frieza's ignorance of Buu's rampage on Earth and his eventual defeat at the hands of Goku and Vegeta isn't a plot hole at all.
This is bolstered by the fact that Kid Buu, the remaining evil part of Majin Buu that was killed via Spirit Bomb, wasn't sent to Hell upon his death. Instead, his soul was reincarnated as the human Uub, who later went off to train with Goku. With Buu's spirit not sent to Hell, there's even less reason to believe that Frieza in the manga's continuity would have known about Majin Buu being released by Babidi in the present day, let alone him fighting Goku.
