Just like The Last Kingdom series, Seven Kings Must Die made some major adjustments to the source material, yet the biggest difference from the original novels actually damages Uhtred's overall story. Both on-screen and in the books, Uhtred is defined by his quest to reclaim his ancestral home of Bebbanburg. Although people and the tides of history occasionally get in the way, he is always pulled back towards the northern fortress and his birthright. However, in changing Uhtred's ending from the books, Seven Kings Must Die alters a key aspect of this journey, making its resolution less powerful.
https://screenrant.com/last-kingdom-movie-seven-kings-book-change-hurt-uhtred-ending/