MCU Rumour: Marvel Have Secretly Cancelled X-Men Dark Phoenix & New Mutants
4. New Mutants - CANNED
According to Slugzilla, New Mutants is a mess because Fox ignored advice about the correct way to approach the material and hired Josh Boone with his darker vision that ended up being a horror movie. Unfortunately for Boone, Fox then allegedly decided they wanted to go with the originally suggested vision and lightened it all up (hence the rumoured extended reshoots).
There have been set issues with actors, whole new characters were added to make it sell better (X-23 and Colossus) and while things were looking more positive, Disney's influence has shelved the project indefinitely. The reshoots apparently came after the test screenings showed the film to be "abysmal" and Fox realised it needed to be saved. But if this is true, it must be beyond saving.
Slugzilla also points out that that is why Marilyn Manson's cover of 'Cry Little Sister' - which was recorded SPECIFICALLY for New Mutants - has been released a year ahead of the film's supposed release date.
As a side bit of information, the leak says that Mr Sinister was never involved.