Who really knows what Mustaf might have done if he was playing significant minutes against the other teams' top players?
Fans have a tendency to get excited about players based on limited playing time in blow-out or garbage time situations.
with no inside information, there are multiple possibilities:
kid got homesick
kid wanted more playing time
some kind of conflict with coaches
just didn't like MN and/or covid restrictions
I hope/assume Pitino was honest with the kid about his prospects for playing time. But kids have a tendency to hear what they want to hear. I see that a lot at the HS level - kids (or their parents) get bent out of shape when the kid does not get the playing time that they think they deserve. Coaches do not want to run off any kids - they need the bodies for practice - but if a kid is not going to play significant minutes on varsity and the coach is clear about the situation, some kids can deal with it and some kids bolt.