Continuity mistake: Despite having the Phoenix power at the end of X-Men: Apocalypse, which manifests around her as a Phoenix firebird and she uses to defeat the titular villain, Jean Grey doesn't actually get merged with the entity until 9 years later in this movie.
Suggested correction: Jean used her full psychic powers in the previous film, not the Phoenix Force. Even without the Phoenix Force she was an incredibly powerful, Omega Level mutant.
The Phoenix firebird is literally shown to be emerging from Jean's body when she kills Apocalypse. She can't both have the Phoenix firebird already within her and then somehow also have it invade her body in this film. The mistake is valid, and I would argue it should be listed as a plot hole more than a continuity error.