At the peak of popularity, Fullmetal Alchemist received a 51-episode anime adaptation in 2003–2004. At the time, though, the manga series was still ongoing. The anime studio Bones had several options while waiting for the manga series to complete. They could have gone the My Hero Academia route of producing story arcs on a seasonal basis to align with the manga or the Bleach route of creating original side stories. Instead, the studio honored Hiromu Arakawa's request. The series pivoted from the manga, only covering seven volumes of the original material and providing a different ending.
Source：https://www.cbr.com/2003-fma-original-ending-worth-watching/
