Of course, Reynolds is also a producer on the movie, so his combined roles should make for a fair amount of power and clout within the project.
When Miller and Reynolds disagreed over the artistic vision and casting of Deadpool 2, Fox ended up siding with the star.
Of course, Fox isn’t totally blameless in this situation.
While not directly related to Tim Miller’s decision to leave Deadpool 2, they were the ones who had final decision in regards to the Miller/Ryan Reynolds feud.