Personally, I wouldn't describe Bosa "slipping," but rather scouts/media have started to study Oregon's DeForest Buckner and NFL teams' needs are becoming more apparent. So it more that Buckner got elevated to Bosa's level, and not Bosa dropping.
Buckner was overshadowed in the regular season because of Oregon's overall piss-poor defense, but their fans will be the first to tell you that Buckner was no way at fault (I've seen a lot of them describe their defense this year as "Buckner plus 10 friends"). If you go back and watch Oregon's games this past season, you'll see that Buckner faced a ton of double and triple-teams as well. It was mostly because Buckner was Oregon's only respectable force on DL, but he still managed to rack up 83 TLKs, 17 TFLs, 10.5 sacks, and 5 PDs as a DE/DT for them.
Bottomline, Bosa and Buckner are both far and away the elite prospects in their position groups, and teams will probably have to flip a coin between the two.