Beerus was asleep when cell, androids, buu were alive because beerus said he was asleep for that time in between goku's birth and battle of gods. Plus beerus could have been asleep when buu was created and went on a rampage, I don't think theirs a plot hole. It makes sence, because majin buu wasn't free for that long until he was sealed away, beerus could have been asleep when buu was on that rampage