Considering Reynolds was the driving force behind the movie being made LONG before Miller was even approached for the project I would say he has a right. Sounds like junkie xl has his own issues with miller leaving. And Miller parted ways pretty amicably with fox and Reynolds. Hes already up for another directing gig for fox so it's not like they had a massive falling out. Reynolds wanted to do things his way, the way that made the first even happen in the first place, and miller didn't feel like it was something he wanted to do anymore. Pretty average actually as someone who works in the film industry.
It is a shame though. Junkie XL has been doing some fantastic work. But it is what it is. As long as Reynolds is deciding more choices than fox is I think the movie will be okay.