Directing credits can be a really tricky thing to nail down.
You only need to look at the ongoing legend about Steven Spielberg and Tobe Hooper over Poltergeist to see that it can have a lasting effect on a film. As you may have already heard, the death of Zack Snyder's daughter, Autumn, caused the director to step away from post-production and reshoots on Justice League - with Joss Whedon stepping in to replace him and guide the film over the finish line.
It's understood that Whedon has since been tasked with making significant reshoots to the film, so much so that it looks like he's getting a directing credit on the film. There's been no official word on the topic from Warner Bros., however GeekTyrant clocked the description of the Justice League trailer on Warner Bros' UK YouTube channel as listing Joss Whedon and Zack Snyder as directors.
That description has since been edited to remove that line, and it's likely that the credits haven't yet been finalised for the film. Ultimately, the decision rests with the Directors Guild Of America, as it's up to them to determine whether or not Whedon gets a credit - and if rumours are to believed, Whedon's doing a lot more work than initially reported.