Justin Bieber says he has NO IDEA why people would think he's a "brat" -- telling TMZ, "I've never heard that rumor before."
Bieber was leaving Katsuya in Hollywood yesterday ... when we asked about a report that he was a total jerk on a recent American Airlines flight from L.A. to Asia ... being difficult with flight attendants and refusing to take a photo with the pilot.
Justin replied, "I don't think you should believe everything you hear ... people make stuff up all the time."
He adds, "Supposedly I'm dead today ... that's the rumor."
Still, we gotta ask ...