It had been announced that Deadpool 2 director Tim Miller left due to creative
differences with Ryan Reynolds. They said that he left on good terms but many reports beg to differ saying they've been butting heads for quite some time now.
Miller may have left due to Cable's casting, wanting to make the sequel more stylistic. Reynolds to maintain the same tone as the first one.
While Miller played a big role in making the first Deadpool film, Reynolds and his writers seem to be adamant in their decision to keep the tone of the sequel the same.