The New Mutants has had a rather chequered history, in that it began production some years ago under Fox.
The movie was filmed from July to September 2017, meaning the actors have come a long way since then, with many having other movies and TV series out in the interim.
It was then decided the film needed reshoots, as the horror-style trailer which was released did not entirely match up with the YA-style movie the director had planned to make.
As a result, the horror genre was decided as the way in which the film would play out, and the 2018 release was pushed back multiple times.
source: express.co.uk