The Promised Neverland Ending, Explained
The anime and the manga end differently, and the difference is a price. In the manga Emma pays for the new Promise with her memory; the anime's finale ends instead on a tearful reunion.
Spoilers throughout. This page assumes you have finished the series.
Two endings, and unusually for this site the gap between them is not a matter of taste. One of them shrinks the price the story was about.
What the manga does
Emma meets the all-powerful being the story calls "Him" and re-forges the Promise, bringing all of the children to the human world. As the price, she is sent to another part of the human world with no memory of her past or her family. The escape succeeds. Norman, Ray and the others eventually find her, and she does not recognize them.
That is a hard ending, and it is the ending the series was built for. The Promised Neverland runs on the idea that getting out costs something, that the children are trading away pieces of themselves at every stage, and the final trade is the largest version of a bargain the story has been making since the first episode.
What the anime does
Season 2 compressed events from chapters 38 to 181 into eleven episodes. After the Promised Forest arc it turned to an anime-original story, omitting several arcs, Goldy Pond among them, and arrived at the finale without most of the material the ending was supposed to land on.
So the bargain changes shape. The children go through the gate to the human world while Emma, Norman and Ray stay behind to forge a new promise with the demons, shown mostly as a slide-show montage, and years later the three return to a tearful reunion. In the accounts of the episode we can check, no memory is lost; the story simply ends on the reunion.
Why that breaks it
Shrinking the cost does not simply shorten the story, it contradicts it. A thriller about children escaping a farm where they are livestock is powered by the sense that the world charges for everything, and the finale answers the question the whole series was asking with a far smaller bill. The plot arrives at the same physical location as the manga and means the opposite thing on arrival.
The complaint about the anime is not that it departed from a perfect original. It is that it kept the destination and discounted the fare.
How this compares to a good divergence
Fullmetal Alchemist's 2003 anime also ran ahead of its manga and invented a conclusion, and it produced something coherent and deliberate that argues with the source on purpose. That is the case for adaptations diverging. The Promised Neverland's second season is the case against doing it in eleven episodes.