It can be an objective fact until it's sufficiently disproven. 3+3=6 is an objective fact, but until it's proven it's not known to be an objective fact. Until it's been proven, it's still in a state of "it can still can be disproven", yet that doesn't change it was still an objective fact all along. 3+3=7 is as not an objective fact, but until it's disproven it's still possible it's an objective fact.
Vegeta wasn't long gone. He'd been gone a few hours. The Buu Saga all happens in around one day.
Krillin is weak so maybe he can't sense the differences between very powerful ki.
Majin Vegeta didn't struggle relying on his memory because Gohan had lost considerable power. Maybe Kid Gohan and Majin Vegeta are quite close in power.
Super Buu and Fat Buu, I already gave reasons why the others don't pick up on it. It's also still possible Piccolo just senses ki better than the others. Did he actually say he can't sense a difference between Goku and Vegeta, or he just neglected to say anything about it?
Gohan, Kaioshin and Kibito felt SSJ3 Goku's power
What's the significance of this?