It's not about the quality of his game, it's about attitude.
If we are only talking about Sevilla then we can say most players had attitude problems in that game as they seemed unmotivated and unfocused until we were 2-0 down and Messi came on.
I think it's unfair to single out players when whole team minds are not in the game but this is somewhat expected when you have 11 points advantage at the top of the table, coming off international break and with important CL game upcoming in 4 days. Combine all of that with playing at one of the toughest stadium not only in La Liga but in the world and you get what we did.
When a team as a group is motivated and playing closer to their potential like against Chelsea then I didn't see Dembele having attitude problems. Yes, he lacks tactical discipline/awareness in defense but that's expected from a young player and I'm sure EV is focusing on improving that aspect of his game. So I don't see attitude problems with Dembele on the field and I absolutely don't know anything about his personal life to know if some (bad) things about him that were written in last months in the media are true and I don't want to speculate on them without knowing more facts. But if there are really some concerns about his off the field attitude then I hope Dembele (with help of the club) will adress those issues seriously if he didn't already.