Dark Phoenix opened in theaters this weekend to less than stellar reviews and the lowest opening weekend box office total ever for one of the X-Men movies ever. On top of all that, the film also has a glaring plot hole that seems to ignore the events, particularly the ending, of X-Men: Apocalypse entirely.
To recap, X-Men: Apocalypse was primarily concerned with the return of the mutant demigod Apocalypse, who turned Magneto, Storm, Psylocke, and Angel into his Four Horsemen and then set about destroying the human world so that he could make a new world in the image of mutants. The film included a subplot about Professor X helping Jean Grey contain a powerful force buried within her that would threaten to consume her during nightmares.
In the finale of Apocalypse, Professor X was forced to reverse course and encourage Jean to release that force from within herself and harness its power to defeat Apocalypse. This tactic proved successful but left the X-Men and X-Men fans to wonder if Professor X and Jean were ready to reckon with the power that they had unleashed.
Jean's hidden power was represented visually by a flaming raptor, the power signature of the Phoenix Force. This was all expected to be setting up a retelling of "The Dark Phoenix Saga" in the next installment of the X-Men movies. This turned out to be correct, and Dark Phoenix was put into development. But the origin of Jean's relationship to the Phoenix Force as told in Dark Phoenix ignores everything established in X-Men: Apocalypse.
Dark Phoenix follows the broad strokes of how Jean Grey became possessed by the Phoenix Force in the original "The Dark Phoenix Saga." The X-Men go on a mission to space. Things go badly. Jean is going to die, but then she makes contact with the Phoenix Force, which bonds to her and gives her the power to survive.
But Dark Phoenix takes place about a decade after the events of X-Men: Apocalypse and offers no explanation as to why Jean was able to wield Phoenix-like power before ever coming into contact with the Phoenix Force in space. It's as if Jean's subplot in X-Men: Apocalypse never happened.
There are ways to retcon this for sure — those powers were innate to Jean and just happened to manifest as a flaming, birdlike entity, or whatever — but no explanation is present in Dark Phoenix. The X-Men movies are known for their inconsistent continuity, and this seems like one more major error before this franchise takes its final bow.